A seat has an occupant detecting function and is installed in a vehicle. The seat includes a seating portion, a capacitance sensor, a controller, and a supporting member. The seating portion is adapted to support buttocks and thighs of an occupant of the vehicle. The seating portion includes a seat cushion therein. The capacitance sensor is attached to the seating portion. The controller is installed to the seating portion and configured to apply a voltage to the sensor and to detect a value of an electric current, which passes through the sensor because of the voltage. The supporting member supports the controller from a lower side thereof in a vertical direction, such that the controller is rotatable about the supporting member when the seating portion is pressed by the occupant.