A signal processor includes a period detection section which detects that a period is currently used for communication of a frame; a pattern detection section which detects, from the received signal, a first signal pattern by which the end of communication of the frame is recognized; and an output processing section which outputs the received signal to a controller, configured to instruct, upon detection of the first signal pattern in the period being currently used for communication of a frame, the controller to halt startup of communication action of the next frame, until the period being currently used for communication of a frame comes to the end, to thereby reduce an event such that frames are transmitted from a plurality of communication devices simultaneously, and to thereby allow the communication action for the next frame to proceed correctly.