PURPOSE: To provide an instruction decoding method which can easily add new instructions even for the same instruction code.

CONSTITUTION: The decoding method of the instruction codes which are read out of a main storage device 1 and stored in an instruction register 2 is switched by an instruction action mode flag 5 of a program executing state register 4. Then the addresses of different microprograms stored in a RAM 7 are generated for the same instruction code, and the instruction is carried out by an executing unit 9. In such a constitution, a flag is set to indicate an instruction decoding method so that different microprogram executing head addresses can be generated for the same instruction code. Thus, it is possible to easily add an instruction having a new function in an existing instruction code system without adding newly an instruction code.