PURPOSE: To execute debugging of a microprogram by simple hardware by connecting a microprogram debugging device to a CPU, so that a clock selects one of the inside and the debugging device.

CONSTITUTION: A debugging device 35 is connected from the outside of a CPU1. A microprogram to be debugged is stored in a processor 36 of the debugging device 35. A switching signal for making a clock line 33 outputted from a processor 36 effective is outputted by the processor 36, to a switching signal line 34 of a selector 31 for switching a clock line 32 outputted from an OSC29 and the clock line 33 outputted from the processor 36. Thereafter, the head micro-instruction code of a microprogram to be debugged, which is stored in advance in the processor 36 is set to a CDR-B37 through a data input line 41 and a set signal line 42, and only one clock is outputted to the clock line 33. In this way, hardware 25 executes a corresponding operation once.