PURPOSE: To realize the CPU run-away detecting system, which can be easily prepared, and a debagging system for a simple program made into a ROM concerning the system for detecting the run-away of a CPU at a microcomputer system.

CONSTITUTION: A flag 33 is provided to show the 1st byte of an instruction code 32 composed of plural bytes transmitted from a source program 31 through an assembler or compiler 3, the address of a 1st byte signal 103 decoding the flag and the instruction code fetched and stored last time is compared with this time fetched address, the branch of a program to any byte excepting for the 1st byte of the instruction is detected according to a program branch detecting signal 104 which is outputted when those addresses are not coincident and in that case, the run-away of the CPU is informed to a CPU 11.