Error connection to server: FATAL: role 'postgres' does not exist

I've found that error when I connect postgres database with pgAdmin3. And I thought postgres username is lost but I've tried psql -l in terminal then I found postgres username still exist.

                         List of databases
   Name    | Owner | Encoding | Collate | Ctype | Access privileges 
-----------+-------+----------+---------+-------+-------------------
 postgres  | user  | UTF8     | C       | UTF-8 | 
 template0 | user  | UTF8     | C       | UTF-8 | =c/user          +
           |       |          |         |       | user=CTc/user
 template1 | user  | UTF8     | C       | UTF-8 | =c/user          +
           |       |          |         |       | user=CTc/user
(3 rows)

Please let me know how to recover postgres issue. Thanks.

via ppshein