An oscillator circuit is provided which employs a single active element. In particular, the circuit utilizes a novel four-terminal semiconductor device which is able to act as a transistor with three of its terminals and, in case of use as the transistor, has the emitter-grounded current amplification factor h.sub.FE controllable with a voltage at its fourth terminal. When the novel four-terminal semiconductor device is connected so as to operate as an emitter-grounded transistor, voltage changes at its output terminal, which corresponds to the collector of the transistor, is the same in phase as voltage changes at its fourth terminal. This fact is utilized in the present invention to cause oscillation by the generative feedback from the output terminal to the fourth terminal.