diff --git a/ssm-agent.sh b/ssm-agent.sh
index 7e08161..613f4e5 100644
--- a/ssm-agent.sh
+++ b/ssm-agent.sh
@@ -1,51 +1,35 @@
 #!/bin/bash
 
-role_name="ssm-bastion-activation"
-default_instance_name="ssm-bastion"
-
-# get AWS API tokens through IRSA
-# see: https://aws.amazon.com/jp/blogs/news/introducing-fine-grained-iam-roles-service-accounts/
-function get_token(){
-    aws sts assume-role-with-web-identity --role-arn $AWS_ROLE_ARN --role-session-name ssm-bastion --web-identity-token file://$AWS_WEB_IDENTITY_TOKEN_FILE --duration-seconds 43200 > /tmp/irp-cred.txt
-    export AWS_ACCESS_KEY_ID="$(cat /tmp/irp-cred.txt | jq -r ".Credentials.AccessKeyId")"
-    export AWS_SECRET_ACCESS_KEY="$(cat /tmp/irp-cred.txt | jq -r ".Credentials.SecretAccessKey")"
-    export AWS_SESSION_TOKEN="$(cat /tmp/irp-cred.txt | jq -r ".Credentials.SessionToken")"
-    rm /tmp/irp-cred.txt
-}
+if [ -z "${BASTION_ROLE_NAME}" ] || [ -z "${BASTION_INSTANCE_NAME}" ]; then
+    echo "ERROR: Some variables are not found. Please set BASTION_ROLE_NAME and BASTION_INSTANCE_NAME." >&2
+    exit 1
+fi
 
 function activate(){
     echo "start activate process ..."
-    expire=$(date -d "10 days" +%Y-%m-%d)
-    aws iam get-role --role-name $role_name > /dev/null  2>&1
-    if [[ $? -ne 0 ]]; then
-        echo "ERROR: IAM role $role_name not found."
-        exit 1
-    fi
-    activation=$(aws ssm create-activation --default-instance-name $default_instance_name --iam-role $role_name --expiration-date $expire --output text)
+    activation=$(aws ssm create-activation --default-instance-name "${BASTION_INSTANCE_NAME}" --iam-role "${BASTION_ROLE_NAME}" --output text)
     SSM_AGENT_CODE=$(echo $activation | cut -f 1 -d ' ')
     SSM_AGENT_ID=$(echo $activation | cut -f 2 -d ' ')
-    echo $SSM_AGENT_ID > /var/run/ssm-agent.id
 }
 
-function deactivate(){
+function shutdown(){
     echo "start shutdown process ..."
+    instance_id=$(cat /var/lib/amazon/ssm/registration | jq -r .ManagedInstanceID)
+    aws ssm deregister-managed-instance --instance-id $instance_id
     kill $(pgrep amazon-ssm)
-    id=$(cat /var/run/ssm-agent.id)
-    echo "delete activation $id"
-    aws ssm delete-activation --activation-id $id
     echo "shutdown process completed."
 }
 
 function start(){
     amazon-ssm-agent -register -code "${SSM_AGENT_CODE}" -id "${SSM_AGENT_ID}" -region "ap-northeast-1"
+    aws ssm delete-activation --activation-id "${SSM_AGENT_ID}"
     amazon-ssm-agent start &
     echo "activate process completed. ssm-agent has started."
 }
 
 ## main process
 
-trap deactivate TERM
-get_token
+trap shutdown TERM
 activate
 start