In order to propose a signal input and output apparatus for changing over control depending on a detection result by discriminating the type of signals when signals of two or more types are passed in the same signal line, a signal input and output apparatus comprises signal discrimination changeover means for discriminating the type of control signal through cable, plug and jack, as a transmission path generating a discrimination signal, supplying it to a controller, and changing over the control of a signal processor to the controller on the basis of the discrimination signal, and thereby input and output of plural control signals can be processed through a single transmission path. As a result, by a single jack only, plural signals can be passed on the same transmission path by optimizing the transmission and reception paths and transmission and reception elements, and therefore the parts mounting area can be saved, and states of various types of signals can be detected.