The present invention pertains to improvements in signal transmitting and receiving equipment for transmitting and receiving signals regarding the M, S and T functions between a numerical controller and a machine tool. Numeric codes (c.sub.l to c.sub.n) that are sent from the NC apparatus to the machine tool are added or combined with a redundant signal (pt) for checking the validity thereof. Only when it is decided that the received codes are valid, based on the added signals, is the machine tool operated. With such an arrangement, even if a parallel transmission system using many signal lines is employed it is possible to offer an NC machine tool which is capable of preventing erroneous operations related to the M, S and T functions, and hence the NC machine tool has high reliability.