To excellently and smoothly perform the work for assembling both of a first and a second assemblies without damaging parts by evacuating a movable roller backward, resisting energizing force of pre-load springs, with relatively simple structure.

A second housing 2 is formed with an assembling through hole 61 formed from outside toward a support shaft 39b. When assembling the second housing 2, in which a high-speed side shaft 17 and the movable roller 38 are previously assembled, to a first housing, in which a low-speed side shaft 3 and an outer ring 32 are previously assembled, it is possible to evacuate the movable roller 38, resisting energizing force of the pre-load springs 47a and 47b by only pushing the support shaft 39b of the movable roller 38, resisting energizing force of the pre-load springs 47a and 47b, while inserting an assembling shaft 62 into the assembling through shaft 61, and interference between the movable roller 38 and the inner peripheral surface of an outer ring 32 can be avoided.