PURPOSE: To dispense with an independent 90° phase shifting circuit so as to simplify the whole circuit by providing an oscillating gate type element circuit with a feedback circuit which limits the amplitude of the output signal of a signal detecting circuit to a fixed, value and feeding it to the gate of an oscillating gate type element (RGT).

CONSTITUTION: A capacitor C is connected, in parallel with a resistor R, between the inverse terminal and the output terminal of the operational amplifier OP constituting a signal detecting circuit 8. And, the drain voltage VD of the RGT1 is fixed to the output voltage of a bias power source VB connected to the noninverse input terminal of the operational amplifier OP The output signal of a circuit 8, where the AC current outputted from RGT1 is converted into AC voltage and the phase is advanced by 90°, is fed back to the gate, being limited to a certain value through a feedback circuit. Hereby, RGT1 continues self-exciting oscillation being stable with natural frequency which changes according to the pressure applied to this.