PURPOSE: To improve the operation efficiency of a microcomputer system by executing a noise as a malfunction, only when an operation failure whose self- recovery is impossible even by an initializing operation is generated, in an MPU of the system.

CONSTITUTION: When an operation failure of a microprocessor MPU 1 is not recovered is spite of a fact that an output Qm-P of a watched timer 2 executes an initializing operation of the MPU 1, and a fact that the number of times of an output of the output Qm-P of the watched timer 2 reaches N is counted by an N-scale number circuit 11, Qn-P of a counting circuit 11 is outputted. This output Qn-l is a malfunction detecting output of the MPU 1, and when the number of times N of counting of the N-scale number counting circuit is selected suitably is advance, it can be decided that an unrecoverable operation failure is generated in the MPU 1. Accordingly, by driving a malfunction display circuit 12 by this Qn-P, a fault of the MPU 1, and also, a microcomputer system is displayed.