PURPOSE: To change the oscillating level and to obtain the stable performance for an AM modulating circuit, by switching a transistor which is inserted into a constant current path to perform a differential operation with a modulated wave and then varying the constant current.

CONSTITUTION: A carrier wave oscillating circuit 9 consists of a differential amplifier 13, and a resonance circuit 12 for decision of carrier frequency is added to the collector of a transistor (TR)T14 of one of a differential pair. A positive feedback is applied to the other TRT13 forming the differential pair from the output terminal of the circuit 12 through a level shifting circuit. At the same time, the current of a constant current source of the amplifier 13 is varied by a modulated wave to vary the oscillating level. In other words, the oscillating level of the carrier wave is decided by a product of the resonance impedance of the circuit 12 and the current I1 flowing through a differential pair of TRT13 and TRT14. Thus a carrier wave F0 receives an AM modulation by a modulated wave f1. As a result, the degree of modulation is never affected by the variance of the level of the carrier wave to ensure the stable performance.