A capacitive physical quantity sensor includes a sensor element and a detecting element. The sensor element includes first and second fixed electrodes facing a movable electrode. A first voltage is applied to the first fixed electrode and a second voltage is applied to the second fixed electrode. The detecting circuit includes a capacitance-voltage conversion circuit, in which an operational amplifier, a capacitor and a switch including a P-channel MOS transistor and a N-channel MOS transistor are disposed. The transistors have a back gate potential, which is approximately equal to an average voltage of the first voltage and the second voltage.