diff --git a/README.md b/README.md
index 981f6fc..9962b0b 100644
--- a/README.md
+++ b/README.md
@@ -32,6 +32,22 @@ go install github.com/Akimon658/ogjson@latest
 
 Then you can use `ogjson` command.
 
+## Advanced usage
+
+### Change `User-Agent`
+
+There are some websites that `ogjson` cannot access by default because of `User-Agent`.
+To avoid it, you can use `user-agent` flag.
+
+```bash
+ogjson -user-agent "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.127 Safari/537.36"
+```
+
+```
+$ curl "http://localhost:8080/?url=https://docs.github.com"
+{"Policy":{"TrustedTags":["meta","link","title"]},"Title":"GitHub.com Help Documentation","Type":"article","URL":{"Source":"https://docs.github.com","Scheme":"https","Opaque":"","User":null,"Host":"docs.github.com","Path":"","RawPath":"","ForceQuery":false,"RawQuery":"","Fragment":"","RawFragment":"","Value":"http://ghdocs-prod.azurewebsites.net:80/en"},"SiteName":"GitHub Docs","Image":[{"URL":"https://assets-git.f3mw1.com/images/modules/open_graph/github-logo.png","SURL":"","Type":"","Width":0,"Height":0,"Alt":""}],"Video":[],"Audio":[],"Description":"Get started, troubleshoot, and make the most of GitHub. Documentation for new users, developers, administrators, and all of GitHub's products.","Determiner":"","Locale":"","LocaleAlt":[],"Favicon":"/assets/cb-803/images/site/favicon.svg"}
+```
+
 ## License
 [MIT](./LICENSE)
 
diff --git a/main.go b/main.go
index 25ee4c5..d4251ef 100644
--- a/main.go
+++ b/main.go
@@ -2,25 +2,50 @@ package main
 
 import (
 	"encoding/json"
+	"flag"
 	"log"
 	"net/http"
 
 	"github.com/otiai10/opengraph"
 )
 
+type handler func(http.ResponseWriter, *http.Request) error
+
+func (f handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if err := f(w, r); err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+	}
+}
+
 func main() {
-	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
-		ogp, err := opengraph.Fetch(r.FormValue("url"))
+	ua := flag.String("user-agent", "Ogjson/1.1", "Value of User-Agent")
+	flag.Parse()
+
+	http.Handle("/", handler(func(w http.ResponseWriter, r *http.Request) error {
+		url := r.FormValue("url")
+		req, err := http.NewRequest("GET", url, nil)
 		if err != nil {
-			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return err
 		}
+		req.Header.Add("User-Agent", *ua)
 
-		w.Header().Set("Content-Type", "application/json")
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			return err
+		}
 
-		if err = json.NewEncoder(w).Encode(ogp); err != nil {
-			http.Error(w, err.Error(), http.StatusInternalServerError)
+		og := opengraph.New(url)
+		if err = og.Parse(resp.Body); err != nil {
+			return err
 		}
-	})
+
+		w.Header().Set("Content-Type", "application/json")
+		if err = json.NewEncoder(w).Encode(og); err != nil {
+			return err
+		}
+
+		return nil
+	}))
 
 	log.Fatal(http.ListenAndServe("0.0.0.0:8080", nil))
 }