PURPOSE: To make the oscillation output level stable by detecting the oscillating state of an oscillated level or phase or harmonics so as to control the loop gain in response to the oscillating state.

CONSTITUTION: An output of an amplifier 1 with a gain K is fed back positively to the amplifier 1 via a phase shifter 2 having a feedback rate of beta. The feedback rate beta is controlled in response to the control input to a control terminal 4 of the phase shifter 2 to change the phase characteristic thereby varying the oscillating frequency. The gain of the amplifier 1 varies with the change in a current of a variable current source 5 as the gain control means. The current of the current source 5 is controlled by a detected output from a level detection circuit 6 and the detection circuit 6 controls the variable current source 5 in response to the output of the detection circuit 6 detecting an output level of the amplifier 1 so as to make the output level stable.