PURPOSE: To detect runaway of a microprocessor, by outputting a runaway signal with a timer set by a decoding output of an address decoder after a specific time.

CONSTITUTION: A microprocessor 10, an address decoder 11 and a memory 12 are connected to a bus line 9. An output 13 of the address decoder 11 is connected to a preset terminal 16 of a counter 15 of a timer 14. A clock signal generator 17 is connected to a count input terminal 18 of the counter 15. A carry output 19 of the counter 15 is outputted as a runaway signal. If the counter 15 is not preset, the counter overflows for a certain time and an overflow signal, i.e. the runaway signal is outputted.