PURPOSE: To make a sufficient safety check without loading a heavy load on a computer by receiving a signal outputted through a computer by a relay, and comparing it with a reference value and discriminate an abnormality by an external circuit.

CONSTITUTION: A reference pattern is outputted from the microcomputer 1 on the basis of a subroutine set in the microcomputer 1. These patterns are patterns 1W4, and timers TM2 and TM3 are Tsec timers. If the microcomputer 1 enters a runaway state and a signal enters the output state of the pattern 2, a circuit B is activated and the timer TM2 operates Tsec later to detect the runaway. Contacts TMa and TMb of the timers TM2 and TM3 are turned on when the runaway is detected, and an interlocking display device opens the contact ER2 of a relay ER indicating abnormality to shut down the microcomputer 1.