A communications device 100, which performs data communication, includes an AC cycle sensor 30 which is connected to a power line 106 supplied with an a.c. voltage and generates a synchronous signal SS at timing of an a.c. voltage waveform AC of the power line 106; a data communicator 10 for performing data communication; and a communications controller 20 which performs communication of a control signal including information showing at least one of a communications device and a communications standard and controls the data communicator 101. When the a.c. voltage supplied to the power line 106 is an N-phase and when the cycle of the a.c. voltage waveform is T, data communication to be performed in a communication period subsequent to the period is controlled on the basis of the control signal included in the period of T/2M on condition that M is a natural multiple of N.