PURPOSE: To improve a processing speed by executing rapidly a corresponding instruction unless the instruction following a mask control instruction is a mask control instruction.

CONSTITUTION: When the instruction following a mask control instruction is not a mask control instruction, an instruction control part 2 sends a signal REIFCH in the state A of a mask control instruction by using a microprogram once the mask control instruction is sent through a pipeline, holds the signal REIFCH in a flip-flop in the state W, and cancels all active states of a central processor in the state of an RS11D23 to perform REIFCH operation. Simultaneously, a specific value is set in a counter 5, which starts counting down. When the state of the RE11D23 is completed, a state PROCESS is entered with a start signal, and the next instruction is executed. If there is an I/O interruption request from a channel processor 8 while the counter 5 counts down to zero, a pipeline control part 4 clears the pipeline and a process control part 3 performs I/O interruption processing.