Hi HyunKyu Chang,
According to the documentation: "Unchanged persistable objects are removed from memory when they aren’t used because they can be reloaded from the Mendix Database when necessary".
With that said, there is a couple of things I can suggest you try. Try either committing the non persistable entity (yes, you can commit the non-persistable entity into memory) to see if that solves your problem of the value being erased from memory.
Otherwise, if that didn't work, you can try associating the non persistable entity with the system entity that is used for the Session to see if that fixes the issue.
I hope that helps you, regards!