A server 1 and client 2 have system clock synchronous means 9, 14 for synchronizing a system clocks 6, 12 to the Internet Standard Time, storing means 4, 11 for arranging and storing a plurality of log-in IDs for authenticating said server 1 and client 2 mutually, and log-in ID changing means 8, 13 for searching repeatedly from the top to the bottom of a plurality of log-in IDs stored in said storing means 4, 11, and updating a log-in ID searched at predetermined intervals as the most current log-in ID every time searching.