To provide a supercharger without causing sliding in high rotation and a driving loss in low rotation, with a simple constitution.

The supercharger 10 has a speed increaser 20 comprising an outer race 27 rotated by an input shaft 11 and eccentrically arranged to an output shaft 12, and a plurality of intermediate rollers 31 to 33 arranged in an annular space becoming dissimilar in the peripheral direction of the output shaft 12 on a width in the radial direction of the output shaft 12 between the output shaft 12 and the outer race 27, and is formed by constituting at least one or more of intermediate rollers 33 as a movable roller movable in the peripheral direction and the radial direction of the output shaft 12. In the speed increaser, the outer race 27 is tightly installed on the outer periphery of the whole intermediate rollers 31 to 33, and an inner peripheral surface of the outer race 27 is elastically repulsively contacted with an outer peripheral surface of the whole intermediate rollers 31 to 33.