diff --git a/README.md b/README.md
index 88e82de..7c809c6 100644
--- a/README.md
+++ b/README.md
@@ -30,7 +30,7 @@ It provides `selfupdate` package.
 ```go
 import (
     "log"
-    "github.com/rhysd/go-github-selfupdate"
+    "github.com/rhysd/go-github-selfupdate/selfupdate"
 )
 
 func doUpdate(version string) {
diff --git a/cmd/selfupdate-example/main.go b/cmd/selfupdate-example/main.go
index eb95f10..4fb30f4 100644
--- a/cmd/selfupdate-example/main.go
+++ b/cmd/selfupdate-example/main.go
@@ -3,13 +3,13 @@ package main
 import (
 	"flag"
 	"fmt"
+	"github.com/rhysd/go-github-selfupdate/selfupdate"
 	"os"
 )
 
 const version = "1.2.3"
 
 func selfUpdate() error {
-	return nil
 	up, err := selfupdate.TryUpdate(version, "go-github-selfupdate", nil)
 	if err != nil {
 		return err
diff --git a/selfupdate/detect.go b/selfupdate/detect.go
new file mode 100644
index 0000000..f3bbb65
--- /dev/null
+++ b/selfupdate/detect.go
@@ -0,0 +1,71 @@
+package selfupdate
+
+import (
+	"context"
+	"fmt"
+	"github.com/blang/semver"
+	"github.com/google/go-github/github"
+	"golang.org/x/oauth2"
+	"net/http"
+	"os"
+	"strings"
+)
+
+// ReleaseDetector is responsible for detecting the latest release using GitHub Releases API.
+type ReleaseDetector struct {
+	verPrefix string
+	api       *github.Client
+	apiCtx    context.Context
+}
+
+// NewDetector crates a new detector instance. It initializes GitHub API client.
+func NewDetector(versionPrefix string) *ReleaseDetector {
+	token := os.Getenv("GITHUB_TOKEN")
+	ctx := context.Background()
+
+	var auth *http.Client
+	if token != "" {
+		src := oauth2.StaticTokenSource(&oauth2.Token{AccessToken: token})
+		auth = oauth2.NewClient(ctx, src)
+	}
+
+	client := github.NewClient(auth)
+	return &ReleaseDetector{versionPrefix, client, ctx}
+}
+
+// DetectLatest tries to get the latest version of the repository on GitHub. 'slug' means 'owner/name' formatted string.
+func (d *ReleaseDetector) DetectLatest(slug string) (ver semver.Version, found bool, err error) {
+	repo := strings.Split(slug, "/")
+	if len(repo) != 2 || repo[0] == "" || repo[1] == "" {
+		err = fmt.Errorf("Invalid slug format. It should be 'owner/name': %s", slug)
+		return
+	}
+
+	rel, res, err := d.api.Repositories.GetLatestRelease(d.apiCtx, repo[0], repo[1])
+	if err != nil {
+		if res.StatusCode == 404 {
+			// 404 means repository not found or release not found. It's not an error here.
+			found = false
+			err = nil
+			log.Println("API returned 404. Repository or release not found")
+		} else {
+			log.Println("API returned an error:", err)
+		}
+		return
+	}
+
+	tag := rel.GetTagName()
+	log.Println("Successfully fetched the latest release. tag:", tag, ", name:", rel.GetName(), ", URL:", rel.GetURL())
+
+	ver, err = semver.Make(strings.TrimPrefix(tag, d.verPrefix))
+	if err == nil {
+		found = true
+	}
+
+	return
+}
+
+// DetectLatest detects the latest release of the slug (owner/repo). verPrefix is a prefix of version in tag name (i.e. 'v' for 'v1.2.3').
+func DetectLatest(slug, verPrefix string) (semver.Version, bool, error) {
+	return NewDetector(verPrefix).DetectLatest(slug)
+}
diff --git a/selfupdate/detect_test.go b/selfupdate/detect_test.go
new file mode 100644
index 0000000..7860433
--- /dev/null
+++ b/selfupdate/detect_test.go
@@ -0,0 +1,67 @@
+package selfupdate
+
+import (
+	"github.com/blang/semver"
+	"os"
+	"testing"
+)
+
+func TestGitHubTokenEnv(t *testing.T) {
+	token := os.Getenv("GITHUB_TOKEN")
+	if token == "" {
+		t.Skip("because $GITHUB_TOKEN is not set")
+	}
+	_ = NewDetector("")
+}
+
+func TestDetectRelease(t *testing.T) {
+	v, ok, err := DetectLatest("rhysd/github-clone-all", "v")
+	if err != nil {
+		t.Fatal("Fetch failed:", err)
+	}
+	if !ok {
+		t.Fatal("Failed to detect latest")
+	}
+	if v.LE(semver.MustParse("2.0.0")) {
+		t.Fatal("Incorrect version:", v)
+	}
+}
+
+func TestInvalidSlug(t *testing.T) {
+	d := NewDetector("v")
+
+	for _, slug := range []string{
+		"foo",
+		"/",
+		"foo/",
+		"/bar",
+		"foo/bar/piyo",
+	} {
+		_, _, err := d.DetectLatest(slug)
+		if err == nil {
+			t.Error(slug, "should be invalid slug")
+		}
+	}
+}
+
+func TestNonExistingRepo(t *testing.T) {
+	d := NewDetector("")
+	v, ok, err := d.DetectLatest("rhysd/non-existing-repo")
+	if err != nil {
+		t.Fatal("Non-existing repo should not cause an error:", v)
+	}
+	if ok {
+		t.Fatal("Release for non-existing repo should not be found")
+	}
+}
+
+func TestNoReleaseFound(t *testing.T) {
+	d := NewDetector("")
+	_, ok, err := d.DetectLatest("rhysd/misc")
+	if err != nil {
+		t.Fatal("Repo having no release should not cause an error:", err)
+	}
+	if ok {
+		t.Fatal("Repo having no release should not be found")
+	}
+}
diff --git a/selfupdate/log.go b/selfupdate/log.go
new file mode 100644
index 0000000..82dfebe
--- /dev/null
+++ b/selfupdate/log.go
@@ -0,0 +1,28 @@
+package selfupdate
+
+import (
+	"io/ioutil"
+	stdlog "log"
+	"os"
+)
+
+var log = stdlog.New(os.Stderr, "", stdlog.Ltime)
+var logEnabled = true
+
+// EnableLog enables to output logging messages in library
+func EnableLog() {
+	if logEnabled {
+		return
+	}
+	logEnabled = true
+	log.SetOutput(os.Stderr)
+}
+
+// DisableLog disables to output logging messages in library
+func DisableLog() {
+	if !logEnabled {
+		return
+	}
+	logEnabled = false
+	log.SetOutput(ioutil.Discard)
+}
diff --git a/selfupdate/log_test.go b/selfupdate/log_test.go
new file mode 100644
index 0000000..91dd600
--- /dev/null
+++ b/selfupdate/log_test.go
@@ -0,0 +1,30 @@
+package selfupdate
+
+import (
+	"testing"
+)
+
+func TestEnableDisableLog(t *testing.T) {
+	defer EnableLog()
+
+	EnableLog()
+	if !logEnabled {
+		t.Fatal("Log should be enabled")
+	}
+	EnableLog()
+	if !logEnabled {
+		t.Fatal("Log should be enabled")
+	}
+	DisableLog()
+	if logEnabled {
+		t.Fatal("Log should not be enabled")
+	}
+	DisableLog()
+	if logEnabled {
+		t.Fatal("Log should not be enabled")
+	}
+	EnableLog()
+	if !logEnabled {
+		t.Fatal("Log should be enabled")
+	}
+}