A supercharger for supercharging intake air to an engine includes a casing which has a suction opening covering a suction-side end of the casing and a discharge opening on the side of the opposite end thereof, and a pair of rotors engaged with each other for compressing intake air introduced from the suction opening to supply the compressed air to the engine via the discharge opening. The casing has therein a communication passage for establishing a communication between the suction opening and the discharge opening. The supercharger also includes a bypass valve for opening and closing the communication passage, and an actuator for actuating the bypass valve.