PURPOSE: To improve the linearity of the center in the S-shaped frequency characteristic curve by constituting two resonance circuits by a coil and a capacitor respectively, forming each coil as a spiral coil by a conductor foil provided additionally to each insulation board and providing additionally a ferrite plate to a 2nd coil.

CONSTITUTION: A 1st resonance circuit 31 is resonated to a higher resonance frequency in the S-shaped frequency characteristic curve and a 2nd resonance circuit 32 is resonated to a lower resonance frequency respectively. A positive portion in an input signal fed to an input terminal 6 is tuned by a 2nd resonance circuit 32 and detected by a diode 18. Then the high frequency component is rejected by a capacitor 24 and a resistor 24 and the result is outputted from an output terminal 7. On the other hand, the negative portion in said input signal is tuned by the 1st resonance circuit 31. Then the signal is subjected to detection by a diode 17 and high frequency component rejection by a capacitor 21 and a resistor 23, and the result is outputted from the output terminal 7. Thus, the linearity at the middle portion of the S-shaped frequency characteristic curve is improved, which is obtained in the relation between the frequency of the input signal and the detection voltage.