An ECU (100, 200) includes a monitoring circuit (2) having a counter (3) for selecting an assignment number (21) of an assignment signal (20), a serial communicating unit (4) for transmitting the assignment signal (20) of the assignment number (21) selected by the counter (3) and to and receiving an answer signal from a microcomputer (1). The ECU also includes a comparing/judging circuit (5) for comparing an answer number (11) with a correct answer number (30) and incrementing the assignment number (21) of the assignment signal selected by the counter (3) by one when both the answers match. This circuit (5) maintains the same assignment number (21) when the answers do not match. The microcomputer (1) performs the operation in conformity with the assignment number (21) of the assignment signal (20) received and transmits the operation result by using an answer signal (10).