PURPOSE: To attain sure and quick detection of runaway of a microprocessor, by providing a parity bit and checking the parity of data read out from the processor actually.

CONSTITUTION: Each data in a memory 2 is provided with 1-bit of parity bit. When the processor 1 changes addresses and reads out data from the memory 2 sequentially, an output of a parity checker 3 is changed alternatey. Further, if bit error takes place due to destroyed data in the memory 2 and noise invasion at the readout to the processor 1, the output of the parity checker 3 may not be changed. This is detected at a coincident circuit 5 and a flip-flop 6 is set. The set output of the flip-flop 6 is error notice, allowing to perform system reset and turning on of display light to the operator.