In a power phase period when in normal operation, switch portions SW2H and SW2L and switch portions SW3H and SW3L are turned ON, respectively, and switch portions SW1H and SW1L are turned OFF. And floating power supply is provided from an electrostatic capacitance element CS to buses A and B, a floating control circuit 4, a transmitter circuit 5, and a receiver circuit 6, respectively. In a data phase period, the switch portions SW1H and SW1L are turned ON, and the switch portions SW2H, SW2L, SW3H, and SW3L are turned OFF. By that manner, the electrostatic capacitance element CS is charged by the power supply of a battery B, and an electrostatic capacitance element CH provides the floating power supply to the floating control circuit 4, the transmitter circuit 5, and the receiver circuit 6, respectively. By this manner, a floating switch unit 7 in which the number of the switch portions is considerably reduced can be configured.