A motion sensor includes a sensor element that outputs a sense signal in response to a motion applied thereto and a sensor circuit that senses the motion based on the sense signal. The sensor circuit includes a sensor-element-signal amplifier that receives the sense signal. The sensor-element-signal amplifier operates switchably between at a normal mode and at a low-noise mode that consumes a larger electric power and produces a smaller noise than the normal mode. This motion sensor senses a small motion and a large motion accurately.