Thank you for your concerning.
I'm running two different SUN server now.
One is Web server for user service and the other is DB server which runs Postgres DB.
What should I do to access to my Postgres DB using PGAMDIN in Web server?
Add a new Server in pgAdmin, and enter the appropriate hostname & port etc. You might need to tweak pg_hba.conf on the server, but I assume that's at least partway correct is your website uses the database already.
Or is it impossible to access to Postgres DB remotely?
No, it's perfectly possible - I imagine the vast majority of users work this way - I certainly do.
Regards, Dave