A duplex-system digital-data transmitting apparatus includes a digital-data signal source which outputs serial data at a signal output end. The level of the data is converted by a first level converter circuit. The data is supplied to an FSK modulator circuit to be FSK-modulated. Carriers are, for example, at 11 MHz and 13 MHz and correspond to digital signals "1" and "0", respectively. The modulated signal is amplified by a driver and is coupled to the human body by a capacitor of about 100 pF. When the human body comes into contact with an electrode, the modulated signal transmitted through the human body is amplified by a pre-amplifier, demodulated by a demodulator circuit, and shaped by a comparator. The level of the shaped signal is converted by a second level converter circuit, and the signal is supplied to a serial input end of a receiving unit. Transmission and reception are performed using a half-duplex system.