PURPOSE: To obtain a coin sensor in which the both-terminal voltage of lower frequency between the both-terminal voltages of a receiving side coil does not become low, and detection precision at the time of detection by this lower frequency does not become low by making the impedance of a transmitting coil at the lowest frequency among the frequencies different from each other nearly equal to the output impedance of an amplifier.

CONSTITUTION: A first oscillator 10 outputs the sine wave signal of 10kHz, and a second oscillator 20 outputs the sine wave signal of 160kHz, and a switching signal generator 30 generates a switching signal composed of the rectangular wave of 1kHz. The transmitting side coil 50 is a coil to flow the sine wave currents of 10kHz and 160kHz, and the impedance of the transmitting side coil 50 at 10kHz is set so as to be nearly equal to the output impedance of the amplifier 40. Besides, the kinds of the frequency of the sine wave current to flow in the transmitting side coil 50 can be three or more. Then, the receiving side coil 60 is opposed to the transmitting side coil 50 through a coil passage P, and receives a signal from the transmitting side coil 50.