PURPOSE: To improve the safety to a malfunction by allowing the execution of the important control signals such as a write instruction, a stop instruction, only in a prescribed program address.

CONSTITUTION: The output of a detecting means 10 is kept at an 'L' level while the program of an area B is executed. Under such conditions, a microcomputer has the malfunction by some reason and a stop signal 6 is outputted at an 'H' level together with the input at one side of an AND gate 12 set an 'L' level respectively. Thus the output of the means 10 is kept at an 'L' level as it is. Thus the execution of the signal 6 is inhibited. While the output of the means 10 is kept at an 'H' level while the program of an area A is executed. Under such conditions, the program works normally and a control circuit 5 outputs the signal 6 at an 'H' level. Thus both signals of the gate 12 are set at 'H' levels and therefore the output of the means 10 is set at an 'H' level. Then a gate 8 stops an internal clock 7 and executes a stop instruction.