A supercharger includes a rotative member that is rotated by an electric motor eccentrically to the center of a casing. A valve is rotatably supported by a periphery of the rotative member. The valve is capable of making contact with a periphery of the rotative member so that an inner periphery of the casing and an outer periphery of the rotative member define an annular space therebetween in a full opening position. The valve is capable of making contact with the inner periphery of the casing to partition the annular space into multiple variable space in a full closing position. Opening degree of the valve is controlled in an intermediate position between the full closing position and the full opening position in accordance with an accelerator position in a low-load operation.