To secure the stable oscillation output regardless of the temperature change or variance of a semiconductor element by linearly detecting the sine wave oscillation signal received from a 1st operational amplifier and supplying the smoothed output of the linear detection to the gate of a junction field-effect transistor(JEET).

A frequency setting element (feedback element) 21 is prepared against the non-inverted input (f) produced from the output of a 1st operational amplifier 1, and the positive feedback is applied to the amplifier 1. In such a constitution, a Wien bridge oscillation circuit is obtained. The output of the amplifier 1 is supplied to the inverted input of a 2nd operational amplifier 17 with the reference voltage applied to the non-inverted output of the amplifier 17 respectively. A linear detection circuit 30 including the amplifier 17 linearly detects the sine wave oscillation signal received from the amplifier 1 at a prescribed amplification degree. The output of the linear detection is smoothed and supplied to the gate of a JFET 4. As a result, the variation of output amplitude of the Wien bridge oscillation circuit can be kept constant.