PURPOSE: To eliminate the need for a wire cord between locations where a signal is transmitted and received and to facilitate the extension by using a pipe or fluid in the piping as a propagation medium of an ultrasonic wave and transmitting a signal between the ultrasonic wave transmitters-receivers installed to two positions of a piping path.

CONSTITUTION: Ultrasonic wave transmitters-receivers 12, 13 are installed to the prescribed location of a pipe 11 such as a water supply pipe. A control signal from a controller 14 is converted into a digital signal by a signal controller 15 and given to an oscillation circuit 17 via an amplifier 16. The ultrasonic wave transmitter-receiver 12 oscillates intermittently an ultrasonic wave in response to the digital signal. The ultrasonic wave propagates through the pipe 11 and a fluid in the pipe 11 and is received by the ultrasonic wave transmitter- receiver 13 installed to the other side. Then the ultrasonic wave signal is converted into an electric signal by a receiver circuit 20 utilizing the LC via a transformer 19 and the signal is amplified by an amplifier 21. Thus, an equipment 23 is controlled freely from a remote location. The pipe 11 such as a water supply pipe or hot water supply pipe and the fluid in the pipe 11 are used as the porpagation media for the ultrasonic wave and the signal is transmitted/received between the ultrasonic wave trasmitters-receivers 12, 13 installed to the one and other sides of the pipe 1. Thus, the cordless configuration is attained.