PURPOSE: To monitor a mounting state of a circuit board without giving an excessive load to a main control part, with regard to mounting/dismounting of the circuit board.

CONSTITUTION: When a circuit board monitoring part 4 detects mounting/ dismounting of a circuit board 2 through a mounting/dismounting detecting line 5, the circuit board monitoring part 4 generates an interruption to a main control part 1 through an interruption signal line 6. The main control part 1 which receives the interruption reads in quickly the contents of a mounting state memory 21 in the circuit board 2 through a signal bus 3. In this mounting state memory 21, a state (in the course of operation, generation of abnormality, etc.) of the circuit board 2 is stored, and the main control part 1 can know mounting/dismounting of the circuit board 2.