PURPOSE: To improve the efficiency of a program memory by decoding only one instruction placed right behind an instruction with skip conditions.

CONSTITUTION: An instruction outputted by an instruction register 1 is decoded by an instruction decoder 4 and an instruction detecting means 2 decides whether or not it is an instruction with skip conditions. When so, the instruction detecting means 2 outputs a reset signal after the operation of the instruction decoder 4 to set a flip-flop 3. Consequently, the instruction decoder 4 is made invalid and an instruction decoder 4' is made valid. When a next instruction is outputted by the instruction register 1 in this state, the instruction is decoded by the instruction decoder 4'. A reset signal is outputted to the flip-flop 3 at the end of the operation of the instruction decoder 4' to make the instruction decoder 4 valid and the instruction decoder 4' invalid, respectively.