PURPOSE: To detect the runaway of the program in a microprocessor, by displaying the running state of the microprocessor with light emitting diodes.

CONSTITUTION: In a read-only memory 2 where the control program for a microprocessor 1 is stored in a no operation area (NOA), one-byte instructions each of which advances a program counter by one step are programmed continuously in the program area. In an area of the processing routine for runaway, the processing routine for runaway by which the microprocessor 1 after runaway is recovered to a prescribed operation is programmed following the area NOA. Thus, both areas are arranged to execute a runaway detecting program.