PURPOSE: To improve stability of a circuit by measuring selectively the output of an amplifier having the second amplification degree in order from the smallest amplification degree.

CONSTITUTION: Amplifiers 10∼12 each are different in constant amplification degree, and are set so that, for instance, the amplification degree of the amplifier 10 is correct in case of transmission from a great distance, and the amplification degree of the amplifier 12 is correct in case of transmission from a narrow space or a short distance. Also, an amplification degree of the amplifier 11 is set to the middle of both of them. In this state, a controller 7 executes a measurement as follows. First of all, among the amplifiers 10∼12, an amplifier receiving a signal first to show its signal in its output is grasped. The amplifier having the second amplification degree in order from the smallest one among them is determined, and thereafter, an observation of the signal is executed by the output of its amplifier until the transmission of its signal is ended.