To improve assembling performance, in a supercharger for incorporating an input shaft, an outer race, intermediate rollers and an output shaft into a speed increaser for fitting a front housing to a rear housing.

The input shaft 11 on the front assembly 20A side and the outer race 27 on the rear assembly 20B side, are closely engaged in these rotational direction, in a process of fitting the front housing 21 and the rear housing 22, by preparing a front assembly 20A for installing the input shaft 11 in the front housing 21 and a rear assembly 20B for installing the intermediate rollers 31 to 33, the output shaft 12 and the outer race 27 in the rear housing 22, in the assembling method of the supercharger 10.