An oscillation device has a resonant tunneling diode formed by interposing a gain medium including a first barrier layer, a quantum well layer and a second barrier layer between a first thickness adjusting layer and a second thickness adjusting layer. The oscillation device also has a switch for switching the polarity of a bias voltage being applied to the resonant tunneling diode. The first thickness adjusting layer and the second thickness adjusting layer have different thicknesses. Thus, a single oscillation device is driven to oscillate with different oscillation frequencies.