A resonant circuit includes a substrate; a MEMS resonator including a fixed electrode and a movable electrode formed above the substrate and having a first terminal and a second terminal, the movable electrode having a movable portion opposing at least a part of the fixed electrode; a first input-output terminal connected to the first terminal connected to one of the fixed electrode and the movable electrode of the MEMS resonator; a second input-output terminal connected to the second terminal connected to an other one of the fixed electrode and the movable electrode of the MEMS resonator; a voltage applying unit supplying a potential to at least the first terminal to apply a bias voltage between the first and the second terminals; and a variable capacitance connected between the first terminal and the first input-output terminal to allow a capacitance value to be changed by a change in a potential difference between opposite ends of the variable capacitance.