diff --git a/internal/howmuch/adapter/repo/sqlc/user.sql b/internal/howmuch/adapter/repo/sqlc/user.sql
index a14d9a3..bd7a095 100644
--- a/internal/howmuch/adapter/repo/sqlc/user.sql
+++ b/internal/howmuch/adapter/repo/sqlc/user.sql
@@ -25,3 +25,8 @@ INSERT INTO "user" (
     email, first_name, last_name, password, created_at, updated_at
 ) VALUES ( $1, $2, $3, $4, $5, $6 )
 RETURNING *;
+
+-- name: GetUserByEmail :one
+SELECT id, email, first_name, last_name, password, created_at, updated_at
+    FROM "user"
+    WHERE email = $1;
diff --git a/internal/howmuch/adapter/repo/sqlc/user.sql.go b/internal/howmuch/adapter/repo/sqlc/user.sql.go
index c32e793..6e9a1da 100644
--- a/internal/howmuch/adapter/repo/sqlc/user.sql.go
+++ b/internal/howmuch/adapter/repo/sqlc/user.sql.go
@@ -11,6 +11,27 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
+const getUserByEmail = `-- name: GetUserByEmail :one
+SELECT id, email, first_name, last_name, password, created_at, updated_at
+    FROM "user"
+    WHERE email = $1
+`
+
+func (q *Queries) GetUserByEmail(ctx context.Context, email string) (User, error) {
+	row := q.db.QueryRow(ctx, getUserByEmail, email)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Email,
+		&i.FirstName,
+		&i.LastName,
+		&i.Password,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
 const insertUser = `-- name: InsertUser :one
 
 INSERT INTO "user" (
diff --git a/internal/howmuch/adapter/repo/user.go b/internal/howmuch/adapter/repo/user.go
index 80a48bc..a273ced 100644
--- a/internal/howmuch/adapter/repo/user.go
+++ b/internal/howmuch/adapter/repo/user.go
@@ -86,3 +86,25 @@ func (ur *userRepository) Create(
 		UpdatedAt: userDB.CreatedAt.Time,
 	}, nil
 }
+
+// GetByEmail if not found, return nil for user but not error.
+func (ur *userRepository) GetByEmail(ctx context.Context, email string) (*model.User, error) {
+	queries := sqlc.New(ur.db)
+	userDB, err := queries.GetUserByEmail(ctx, email)
+	if errors.Is(err, pgx.ErrNoRows) {
+		// No query error, but user not found
+		return nil, nil
+	} else if err != nil {
+		return nil, err
+	}
+
+	return &model.User{
+		ID:        int(userDB.ID),
+		Email:     userDB.Email,
+		FirstName: userDB.FirstName,
+		LastName:  userDB.LastName,
+		Password:  userDB.Password,
+		CreatedAt: userDB.CreatedAt.Time,
+		UpdatedAt: userDB.CreatedAt.Time,
+	}, nil
+}
diff --git a/internal/howmuch/usecase/repo/testuser.go b/internal/howmuch/usecase/repo/testuser.go
index c108c68..eea2eab 100644
--- a/internal/howmuch/usecase/repo/testuser.go
+++ b/internal/howmuch/usecase/repo/testuser.go
@@ -27,8 +27,11 @@ import (
 
 	"git.vinchent.xyz/vinchent/howmuch/internal/howmuch/model"
 	"github.com/pkg/errors"
+	"golang.org/x/crypto/bcrypt"
 )
 
+var UserTestDummyErr = errors.New("dummy error")
+
 type TestUserRepository struct{}
 
 func (tur *TestUserRepository) Create(
@@ -46,3 +49,21 @@ func (tur *TestUserRepository) Create(
 
 	return &user, nil
 }
+
+func (ur *TestUserRepository) GetByEmail(ctx context.Context, email string) (*model.User, error) {
+	hashedPwd, _ := bcrypt.GenerateFromPassword([]byte("strongHashed"), 12)
+	switch email {
+	case "a@b.c":
+		return &model.User{
+			ID:       123,
+			Email:    "a@b.c",
+			Password: string(hashedPwd),
+		}, nil
+	case "query@error.com":
+		return nil, UserTestDummyErr
+	case "inexist@error.com":
+		return nil, nil
+	}
+
+	return nil, UserTestDummyErr
+}
diff --git a/internal/howmuch/usecase/repo/user.go b/internal/howmuch/usecase/repo/user.go
index ea2abf0..5693b3a 100644
--- a/internal/howmuch/usecase/repo/user.go
+++ b/internal/howmuch/usecase/repo/user.go
@@ -30,4 +30,5 @@ import (
 
 type UserRepository interface {
 	Create(ctx context.Context, transaction interface{}, u *model.User) (*model.User, error)
+	GetByEmail(ctx context.Context, email string) (*model.User, error)
 }
diff --git a/internal/howmuch/usecase/usecase/user.go b/internal/howmuch/usecase/usecase/user.go
index 938ca40..0f3cb30 100644
--- a/internal/howmuch/usecase/usecase/user.go
+++ b/internal/howmuch/usecase/usecase/user.go
@@ -24,6 +24,7 @@ package usecase
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"regexp"
@@ -35,11 +36,23 @@ import (
 	"golang.org/x/crypto/bcrypt"
 )
 
-var UserExisted = &errno.Errno{
-	HTTP:    http.StatusBadRequest,
-	Code:    errno.ErrorCode(errno.FailedOperationCode, "UserExisted"),
-	Message: "email already existed.",
-}
+var (
+	UserExisted = &errno.Errno{
+		HTTP:    http.StatusBadRequest,
+		Code:    errno.ErrorCode(errno.FailedOperationCode, "UserExisted"),
+		Message: "email already existed.",
+	}
+	UserNotExist = &errno.Errno{
+		HTTP:    http.StatusBadRequest,
+		Code:    errno.ErrorCode(errno.ResourceNotFoundCode, "UserNotExist"),
+		Message: "user does not exists.",
+	}
+	UserWrongPassword = &errno.Errno{
+		HTTP:    http.StatusBadRequest,
+		Code:    errno.ErrorCode(errno.AuthFailureCode, "UserWrongPassword"),
+		Message: "wrong password.",
+	}
+)
 
 type userUsecase struct {
 	userRepo repo.UserRepository
@@ -48,6 +61,7 @@ type userUsecase struct {
 
 type User interface {
 	Create(ctx context.Context, u *model.User) (*model.User, error)
+	Exist(ctx context.Context, u *model.User) (bool, error)
 }
 
 func NewUserUsecase(r repo.UserRepository, d repo.DBRepository) User {
@@ -98,3 +112,24 @@ func (uuc *userUsecase) Create(ctx context.Context, u *model.User) (*model.User,
 
 	return user, nil
 }
+
+func (uuc *userUsecase) Exist(ctx context.Context, u *model.User) (bool, error) {
+	got, err := uuc.userRepo.GetByEmail(ctx, u.Email)
+	// Any query error?
+	if err != nil {
+		return false, err
+	}
+
+	// User exists?
+	if got == nil {
+		return false, UserNotExist
+	}
+
+	// Password correct?
+	err = bcrypt.CompareHashAndPassword([]byte(got.Password), []byte(u.Password))
+	if errors.Is(err, bcrypt.ErrMismatchedHashAndPassword) {
+		return false, UserWrongPassword
+	}
+
+	return true, nil
+}
diff --git a/internal/howmuch/usecase/usecase/user_test.go b/internal/howmuch/usecase/usecase/user_test.go
index 93a1263..70cfe89 100644
--- a/internal/howmuch/usecase/usecase/user_test.go
+++ b/internal/howmuch/usecase/usecase/user_test.go
@@ -63,3 +63,38 @@ func TestCreateUser(t *testing.T) {
 		assert.EqualError(t, err, UserExisted.Error())
 	})
 }
+
+func TestUserExist(t *testing.T) {
+	testCases := []struct {
+		Name   string
+		User   *model.User
+		ExpErr error
+		ExpRes bool
+	}{
+		{"user exists", &model.User{
+			Email:    "a@b.c",
+			Password: "strongHashed",
+		}, nil, true},
+		{"query error", &model.User{
+			Email:    "query@error.com",
+			Password: "strongHashed",
+		}, repo.UserTestDummyErr, false},
+		{"user doesn not exist", &model.User{
+			Email:    "inexist@error.com",
+			Password: "strongHashed",
+		}, UserNotExist, false},
+		{"wrong password", &model.User{
+			Email:    "a@b.c",
+			Password: "wrongHashed",
+		}, UserWrongPassword, false},
+	}
+
+	for _, tst := range testCases {
+		ctx := context.Background()
+		userUsecase := NewUserUsecase(&repo.TestUserRepository{}, &repo.TestDBRepository{})
+
+		got, err := userUsecase.Exist(ctx, tst.User)
+		assert.ErrorIs(t, err, tst.ExpErr)
+		assert.Equal(t, tst.ExpRes, got)
+	}
+}