PURPOSE: To execute the self-diagnosis of a printed board in a short time by storing a diagnostic program in a ROM of a microcomputer in advance or replacing it with a ROM storing a diagnostic program to execute the diagnostic program.

CONSTITUTION: In one set of microcomputers coupled together, one is used as a master microcomputer and the other is a subordinate microcomputer, the subordinate computer is controlled by the master computer with the diagnostic program stored in advance in the ROM to diagnose the normality of the data bus between both the microcomputers and the computers themselves. When a test command from the outside of the printed circuit board 6 is inputted to the microcomputer 1, a call signal is given to the microcomputer 2. The data bus is switched into the output mode, call response data is set to the bus and the input of the call response data is requested to the microcomputer 1.