PURPOSE: To grasp the generating state of machine lock by inputting respective signals on a bus line under control by a sub-CPU at the generation of machine lock and displaying their contents.

CONSTITUTION: A main CPU 1, a program ROM, a control RAM 2, an I/O port 3, and a console 4 are connected to a bus line BUS. A machine lock detecting circuit 5 detects machine locking status due to a misprogram or the like and turns a sub-CPU 7 always kept at reset status to operating status. The CPU 7 inputs respective signals on the bus line BUS at the generation of machine lock and displays the signals on a console 4.