PURPOSE: To realize the monitor process without giving any effect to the program execution by burying the improper order code into the monitor part to the execution program in order to secure an optional selection of the monitor part.

CONSTITUTION: Head order code D2 of execution program 10 is substituted with improper order code E5. When program 10 is carried out, code E5 is set to order code register 25. According to the contents of register 25, the execution routine address is produced at address production circuit 23 and improper order control circuit 24 is controlled to drive monitor circuit 43. After the monitor action, order code conversion circuit 5 is driven to give the original proper order code corresponding to the improper order code. Thus register 25 is set again to drive each order execution circuit 41. When FF61 is under the reset mode, improper order code process circuit 42 is driven to make the program monitor ineffective.