diff --git a/AzureKeyVault.LetsEncrypt/Internal/RetriableActivityException.cs b/AzureKeyVault.LetsEncrypt/Internal/RetriableActivityException.cs
new file mode 100644
index 00000000..d977177f
--- /dev/null
+++ b/AzureKeyVault.LetsEncrypt/Internal/RetriableActivityException.cs
@@ -0,0 +1,25 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace AzureKeyVault.LetsEncrypt.Internal
+{
+    [Serializable]
+    public class RetriableActivityException : Exception
+    {
+        public RetriableActivityException()
+        {
+        }
+
+        public RetriableActivityException(string message) : base(message)
+        {
+        }
+
+        public RetriableActivityException(string message, Exception inner) : base(message, inner)
+        {
+        }
+
+        protected RetriableActivityException(SerializationInfo info, StreamingContext context) : base(info, context)
+        {
+        }
+    }
+}
diff --git a/AzureKeyVault.LetsEncrypt/SharedFunctions.cs b/AzureKeyVault.LetsEncrypt/SharedFunctions.cs
index d92bb425..478b5363 100644
--- a/AzureKeyVault.LetsEncrypt/SharedFunctions.cs
+++ b/AzureKeyVault.LetsEncrypt/SharedFunctions.cs
@@ -54,7 +54,7 @@ public static async Task IssueCertificate([OrchestrationTrigger] DurableOrchestr
                 var result = await context.CallActivityAsync<ChallengeResult>(nameof(Dns01Authorization), authorization);
 
                 // Azure DNS で正しくレコードが引けるか確認
-                await context.CallActivityWithRetryAsync(nameof(CheckIsDnsRecord), new RetryOptions(TimeSpan.FromSeconds(10), 6), result);
+                await context.CallActivityWithRetryAsync(nameof(CheckDnsChallenge), new RetryOptions(TimeSpan.FromSeconds(10), 6), result);
 
                 challenges.Add(result);
             }
@@ -197,8 +197,8 @@ public static async Task<ChallengeResult> Dns01Authorization([ActivityTrigger] D
             };
         }
 
-        [FunctionName(nameof(CheckIsDnsRecord))]
-        public static async Task CheckIsDnsRecord([ActivityTrigger] DurableActivityContext context, ILogger log)
+        [FunctionName(nameof(CheckDnsChallenge))]
+        public static async Task CheckDnsChallenge([ActivityTrigger] DurableActivityContext context, ILogger log)
         {
             var challenge = context.GetInput<ChallengeResult>();
 
@@ -212,27 +212,13 @@ public static async Task CheckIsDnsRecord([ActivityTrigger] DurableActivityConte
             // レコードが存在しなかった場合はエラー
             if (txtRecords.Length == 0)
             {
-                throw new InvalidOperationException($"{challenge.DnsRecordName} did not resolve.");
+                throw new RetriableActivityException($"{challenge.DnsRecordName} did not resolve.");
             }
 
             // レコードに今回のチャレンジが含まれていない場合もエラー
             if (!txtRecords.Any(x => x.Text.Contains(challenge.DnsRecordValue)))
             {
-                throw new InvalidOperationException($"{challenge.DnsRecordName} value is not correct.");
-            }
-        }
-
-        [FunctionName(nameof(AnswerChallenges))]
-        public static async Task AnswerChallenges([ActivityTrigger] DurableActivityContext context, ILogger log)
-        {
-            var challenges = context.GetInput<IList<ChallengeResult>>();
-
-            var acme = await CreateAcmeClientAsync();
-
-            // Answer の準備が出来たことを通知
-            foreach (var challenge in challenges)
-            {
-                await acme.AnswerChallengeAsync(challenge.Url);
+                throw new RetriableActivityException($"{challenge.DnsRecordName} value is not correct.");
             }
         }
 
@@ -247,8 +233,8 @@ public static async Task CheckIsReady([ActivityTrigger] DurableActivityContext c
 
             if (orderDetails.Payload.Status == "pending")
             {
-                // pending の場合は何もしない
-                throw new InvalidOperationException("ACME domain validation is pending.");
+                // pending の場合はリトライする
+                throw new RetriableActivityException("ACME domain validation is pending.");
             }
 
             if (orderDetails.Payload.Status == "invalid")
@@ -266,10 +252,25 @@ public static async Task CheckIsReady([ActivityTrigger] DurableActivityContext c
                     }
                 }
 
+                // invalid の場合は最初から実行が必要なので失敗させる
                 throw new InvalidOperationException("Invalid order status. Required retry at first.");
             }
         }
 
+        [FunctionName(nameof(AnswerChallenges))]
+        public static async Task AnswerChallenges([ActivityTrigger] DurableActivityContext context, ILogger log)
+        {
+            var challenges = context.GetInput<IList<ChallengeResult>>();
+
+            var acme = await CreateAcmeClientAsync();
+
+            // Answer の準備が出来たことを通知
+            foreach (var challenge in challenges)
+            {
+                await acme.AnswerChallengeAsync(challenge.Url);
+            }
+        }
+
         [FunctionName(nameof(FinalizeOrder))]
         public static async Task FinalizeOrder([ActivityTrigger] DurableActivityContext context, ILogger log)
         {