A signal transmitting apparatus comprising: a sending coil configured to be driven by an inputted signal; a receiving coil configured to output a received signal in response to the sending coil being driven; a preprocessing circuit configured such that the received signal outputted by the receiving coil is inputted to the preprocessing circuit; and a detecting circuit (47, 52) configured to detect the inputted signal by a signal outputted by the preprocessing circuit, wherein the sending coil comprises a first conductive part having a first number of windings, the first conductive part being disposed on a first plane, the receiving coil comprises a second conductive part having a second number of windings that is larger than the first number of windings, the second conductive part being disposed on a second plane, the sending coil and the receiving coil are configured so that a peak frequency of the received signal outputted from the receiving coil exceeds a frequency range of a signal that the detecting circuit can receive by adjusting outer diameters of the sending coil and the receiving coil, and a peak voltage of the received signal outputted from the receiving coil exceeds a withstand voltage of the detecting circuit by changing the ratio of the number of windings of the sending coil and that of the receiving coil, and the preprocessing circuit is configured to convert the received signal outputted from the receiving coil into a converted signal and output the converted signal, a frequency of the converted signal being within the frequency range of a signal that the detecting circuit can receive, and a voltage of the converted signal being equal to or lower than the withstand voltage of the detecting circuit. a sending coil configured to be driven by an inputted signal; a receiving coil configured to output a received signal in response to the sending coil being driven; a preprocessing circuit configured such that the received signal outputted by the receiving coil is inputted to the preprocessing circuit; and a detecting circuit (47, 52) configured to detect the inputted signal by a signal outputted by the preprocessing circuit, wherein the sending coil comprises a first conductive part having a first number of windings, the first conductive part being disposed on a first plane, the receiving coil comprises a second conductive part having a second number of windings that is larger than the first number of windings, the second conductive part being disposed on a second plane, the sending coil and the receiving coil are configured so that a peak frequency of the received signal outputted from the receiving coil exceeds a frequency range of a signal that the detecting circuit can receive by adjusting outer diameters of the sending coil and the receiving coil, and a peak voltage of the received signal outputted from the receiving coil exceeds a withstand voltage of the detecting circuit by changing the ratio of the number of windings of the sending coil and that of the receiving coil, and the preprocessing circuit is configured to convert the received signal outputted from the receiving coil into a converted signal and output the converted signal, a frequency of the converted signal being within the frequency range of a signal that the detecting circuit can receive, and a voltage of the converted signal being equal to or lower than the withstand voltage of the detecting circuit.