In processing a sequence program for a b-contact, an input signal is logically inverted ultimately, and the processing for the b-contact requires logical inversion processing which is not involved in processing an a-contact. A logical inversion circuit (8) is provided for logically inverting an input signal corresponding to b-contact designation of the sequence program before the input signal is fed to a central processing unit (3), so that inversion processing in the central processing unit can be dispensed with and operation speed can be increased. The application of a logical inversion signal to the logical inversion circuit can readily be effected by utilizing an address line of the central processing unit.