PURPOSE: To obtain a runaway detecting circuit for microcomputer which can be produced and converted easily into an IC by using the 1st or 2nd abnormal signal delivered from the 1st or 2nd counter circuit to detect the program runaway.

CONSTITUTION: When a program to be executed by a microcomputer 1 has a runaway state, the time Th during which a signal 5 to be monitored is kept at an H level or a period T becomes longer than the initial prescribed value. Thus the count value of the 1st or 2nd counter circuit 11 or 2 reaches the set value N1 or N2. Then the 1st or 2nd abnormal signal 9d or 10d is delivered from the circuit 11 or 12 together with an output signal 13a delivered from an OR circuit 13. Thus the runaway state of the program is detected by a signal 13a. No CR circuit is applied to such a detecting circuit consisting of a logic circuit. Thus this runaway detecting circuit can be easily produced and converted into an IC.