To assemble both first and second assemblies suitably and smoothly without damaging a part at all with a comparatively simple structure, while retrieving a moving roller against the force loaded with a precompression spring.

An outer ring 32 has a tapered section TA formed at the corner at the inner surface. When the second assembly SH, in which a shaft 17 at high speed side and a moving roller 38 are imposed beforehand, is imposed to the first assembly FH, in which a shaft 3 at low speed side and the outer ring 32 are imposed beforehand, the first and second assemblies FH, SH are approached each other so that the moving roller 38 can be pressed against the force loaded with pre-compression springs 47a, 47b. The tapered section TA of the outer ring 32 is brought in contact with the corner of the moving roller 38. Therefore, the moving roller 38 can be retrieved against the force loaded with the pre-compression springs 47a, 47b, and the interference with the moving roller 38 and the inner surface of the outer ring 32 can be avoided.