PURPOSE: To prevent the influence of a fault during the execution of a compound instruction without extending arithmetic time by providing a temporary storage means which is set in case of the occurrence of a fault and reset on the completion of an instruction, and a gate means which is turned on and off by the output of said means.

CONSTITUTION: When the cause of an interruption against which operation need be protected is caused and an interruption mask is not set, a flip-flop F/F is set by the output of a gate G0. Once the F/F is set, operation protecting gates G1W G3 are closed by its output. Consequently, an arithmetic indication signal for indicating the contents of arithmeitc to be performed by an arithmetic circuit, a write destination indication signal for indicating a register where an arithmetic result, etc., is to be written, and a control signal for indicating the operation of an external device, etc. which are all some of signals from a register CMIR are inhibited from being transmitted, thus preventing the influence of the fault.