A communication system has a first communication device, and a second communication device that conducts wireless communication with the first communication device. The first communication device has a first transmitter that transmits a signal to the second communication device, and a first transmission controller that controls the first transmitter. The second communication device has a first receiver that receives the signal from the first communication device, a measurement part that measures a reception intensity of the signal received by the first receiver, and a determination part that determines whether the signal received by the first receiver is a normal signal. The first transmission controller divides a predetermined portion of a predetermined signal into a plurality of portions when transmitting the predetermined signal.