PURPOSE: To enable to accurately transmit a signal by electromagnetically transmitting various switch signals provided at a steering wheel from a signal processor and the steering wheel side to a stationary side through a coupler, thereby preventing the improper contact.

CONSTITUTION: An ON/OFF signal of switch group 1 provided at a steering wheel is encoded by an encoder 2, fed through a coupler 3 to a decoder 4 of stationary side, demodulated and ON or OFF is detected. This coupler 3 is composed in a transformer manner oppositvely of a movable side coil 8 wound through a supporting member 7 around a steering wheel shaft 6 and a stationary side coil 11 wound outside of the stationary unit 10 for rotatably supporting the shaft 6 through a bearing 9, and an encoded signal from the encoder 2 is transmitted to the stationary side without influence to the rotation of the steering wheel 5. Accordingly, improper contacts or noise are not generated, the signals can be accurately transmitted, many switches can be mounted of the steering wheel, and the safety of the operation can be enhanced.