When I restart instance, SSM is connected. but after a few minute, SSM isn't connected.(Target Not Connected)

Please refer to the troubleshooting steps listed here for TargetNotConnected

https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager-troubleshooting.html#ssh-target-not-connected

Could you please check if SSM agent is installed. I am not sure which OS you are using. Please also make sure you are using AWS CLI v2.

Hence here is the link that might help https://docs.aws.amazon.com/systems-manager/latest/userguide/ssm-agent.html -- [1]

Check if all the steps are performed and also if the SSM installed on the instance is not out of date. You can discover this by trying to start the session from Systems Manager in the AWS console; basically going to Systems Manager->Fleet Manager->{INSTANCE_ID}->Instance Actions->Start Session It will throw an error if it is out of date.

https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager-getting-started.html -- [2]

As mentioned in the link below, this error is also returned if you attempt to start a session on a managed node that is located in a different AWS account or AWS Region.

https://docs.aws.amazon.com/systems-manager/latest/userguide/session-manager-troubleshooting.html#ssh-target-not-connected [3]

Once you verify the same, please check if the instances have the proper outbound connectivity? (Outbound to some AWS services including ec2messages.region.amazonaws.com, ssm.region.amazonaws.com, ssmmessages.region.amazonaws.com