PURPOSE: To prevent the misrecognition of the CPU stop at normal working of the CPU by outputting a fact that the CPU has the overflow and stops after a counter worked for a fixed time if no interruption answer signal is sent back to the counter from the CPU.

CONSTITUTION: A counter C starts its counting action again every time the counter is reset by an interruption answer signal 3. Then an interruption is applied to a CPU A from a flip-flop B in timing T25. If the CPU A sends back no signal 3 to an answer, the counter C working at the rise of a clock 1 has the overflow at T26 later than T25 by a fixed time and outputs a signal via a signal line 4 to show that a CPU of a device is stopped. Thus, it is possible to prevent the misrecognition of the CPU stop even though the CPU A is nor mally working.