To attain data transfer, even when the gap between coils is relatively large.

A bias voltage equivalent to a forward voltage VF of a diode 17a is impressed from a bias circuit 17 to one edge (a node N12) of a reception side coil 13. Thus, the potential of a node N11 can be increased, and the threshold voltage of an AND 15 or more can be easily obtained by a voltage induced from an electromagnetically connected transmission side coil 43 to the coil 13. Therefore, a gap between the coils 13 and 43 can be increased at data transfer, while when an oscillation signal CK is stopped at a power supply part 30A, the operation of a mono-multiable 37 is stopped, and the output signal of a buffer 39 is turned into an 'L' level. Thus, NMOS 34a and 34b can be turned into an off state and prevented from being destroyed.