PURPOSE: To attain accurate measurement and to simplify circuit constitution by adjusting an offset so as to obtain a voltage corresponding to the upper limit voltage and the lower limit voltage of an oscillation circuit while interlocking with the adjustment, thereby making the amplitude of the oscillation circuit constant.

CONSTITUTION: The variable resistor VR1 of a noninverting amplifier 42 is adjusted so that the output of a differential amplifier 25 reached a prescribed value when a detection coil Zb and a correction coil Za have a same impedance (Zb=Za) to decide the output voltage e5 of a voltage division circuit 44. The output voltages e5, e2-e4 are decided by the resistance ratio of resistors R33, R34, R35 and R36, the upper limit, the lower limit and the median of the amplitude of an integration oscillation circuit 10 are decided to adjust a sinusoidal wave AC voltage eOSC being an output of the integration oscillation circuit 10. Then the amplitude of the sinusoidal wave AC voltage eOSC being an output of the integration oscillation circuit 10 is kept constant and accurate measurement is attained.