To provide a method by which a board address can automatically be obtained.

A system for extending functions by connecting a microcomputer and a plurality of interface boards encodes the presence/absence of the interface boards as an interface identification code 5 at some address 4 in advance and this code is read to check the presence/absence, the address, etc., of the interface boards to automatically recognize the interface boards. When the addresses are overlapped depending on the case, the addresses, etc., are corrected and obtained.