A transmitter (2) modulates information to be transmitted with an alternating-current signal having a predetermined frequency and includes a variable reactance section (16) which produces resonance with a stray capacitance (18) between a circuit ground (17) in the transmitter (2) and an earth ground (20) which strays from the earth ground (20) and a stray capacitance (19) between the human body (3) and the earth ground (20). A receiver (1) that receives the induced electric field from the transmitter (2) includes an electrode (12) that transmits/receives the electric field to/from the human body (3); a detection signal source (10) that applies a detection signal to the electrode (12); and a detecting section (9) which detects the detection signal, and further detects, based on an amplitude of the detection signal, a resonant state of the variable reactance section (16) in the transmitter (2) held by the human body (3) close to the electrode (12), the stray capacitance (19) between the human body (3) and the earth ground (20), and the stray capacitance (18) between the ground (17) of the transmitter (2) and the earth ground (20), and outputs a signal when the resonant state is present.