To remarkably improve assembling workability even in the case the energizing force of a preload spring is large.

An outer ring 32 (low speed side) is rotatably supported to a housing 1, and a high speed side shaft 17 is rotatably supported eccentrically to the outer ring 32. Two guide rollers 37a, 37b, and a movable roller 38 moving when transmitting torque, are interposed between the outer ring 32 and the high speed side shaft 17. In normal rotation, the movable roller 38 moves in a direction of biting into a wedge between the high speed side shaft 17 and the outer ring 32 and generates pressing force to transmit torque, while in reverse rotation, the movable roller 38 moves in a direction of separating from the wedge and makes the pressing force zero to stop torque transmission. A screw type presser member 58 is provided for adjusting the energizing force of the preload spring 47b.