PURPOSE: To reduce the size and weight and facilitate the handling and installa tion as an automatic switch for various devices by detecting an object which moves in a detectable area.

CONSTITUTION: An induced noise signal which varies as the object (human) moves is received by an antenna 2 and extracted as a variation signal which is generated as a result of the movement of the object (human) by an amplifier 3, a low-pass filter 4, and an amplifier 5 and a music generating device 7 plays music with the output signal of an output circuit 6 by reacting to the motion of the object (human). The music generating circuit is replaceable with a motor to put a body in motion or with a relay to obtain a switch for illumination, power, alarming, voicing, etc.