diff --git a/cmd/acme/cert.go b/cmd/acme/cert.go
index d1643d9..e6c19b9 100644
--- a/cmd/acme/cert.go
+++ b/cmd/acme/cert.go
@@ -93,6 +93,18 @@ func runCert(args []string) {
 		fatalf("no key found for %s", uc.URI)
 	}
 
+	// read crt if existent
+	certPath := sameDir(certKeypath, cn+".crt")
+	certCrt, err := readCrt(certPath)
+	if err == nil {
+		// do not re-issue certificate if it's not about to expire in less than three weeks
+		expiresIn := certCrt.NotAfter.Sub(time.Now())
+		if expiresIn > 24*7*3*time.Hour {
+			errorf("cert is still valid for more than a three weeks, not renewing")
+			exit()
+		}
+	}
+
 	// read or generate new cert key
 	certKey, err := anyKey(certKeypath, true)
 	if err != nil {
@@ -135,10 +147,9 @@ func runCert(args []string) {
 	logf("cert url: %s", curl)
 	var pemcert []byte
 	for _, b := range cert {
-		b = pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: b})
+		b = pem.EncodeToMemory(&pem.Block{Type: x509PublicKey, Bytes: b})
 		pemcert = append(pemcert, b...)
 	}
-	certPath := sameDir(certKeypath, cn+".crt")
 	if err := ioutil.WriteFile(certPath, pemcert, 0644); err != nil {
 		fatalf("write cert: %v", err)
 	}
diff --git a/cmd/acme/config.go b/cmd/acme/config.go
index c1e4179..1e668ef 100644
--- a/cmd/acme/config.go
+++ b/cmd/acme/config.go
@@ -39,6 +39,7 @@ const (
 
 	rsaPrivateKey = "RSA PRIVATE KEY"
 	ecPrivateKey  = "EC PRIVATE KEY"
+	x509PublicKey = "CERTIFICATE"
 )
 
 // configDir is acme configuration dir.
@@ -122,6 +123,21 @@ func readKey(path string) (crypto.Signer, error) {
 	}
 }
 
+func readCrt(path string) (*x509.Certificate, error) {
+	b, err := ioutil.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	d, _ := pem.Decode(b)
+	if d == nil {
+		return nil, fmt.Errorf("no block found in %q", path)
+	}
+	if d.Type != x509PublicKey {
+		return nil, fmt.Errorf("%q is unsupported", d.Type)
+	}
+	return x509.ParseCertificate(d.Bytes)
+}
+
 // writeKey writes k to the specified path in PEM format.
 // If file does not exists, it will be created with 0600 mod.
 func writeKey(path string, k *ecdsa.PrivateKey) error {