PURPOSE: To decrease a thrust force by arranging a roller member to be free to move in the shaft direction by way of providing a shaft direction guide groove in the inner periphery of an outward coupling member and by holding a ball by means of providing a ball holding hole facing against an inward coupling member and the roller member.

CONSTITUTION: A roller member 2 with a first ball holding hole 25 is arranged free to move in a guide groove 12 of an outward coupling member 1. A second ball holding hole 36 is formed on a base part 34 of an inward coupling member 3 so that it is to face against the first ball holding hole 25, and a ball 4 is held by the first and the second ball holding holes 25, 36. When the outward coupling member 1 rotates, the inward coupling member 3 rotates through each of the balls 4. The roller member 2 is guided by a guide surface 14 and rolls and moves as it slides with the ball 4, and a thrust force is decreased as sliding of the inward coupling member 3 in the shaft direction is allowed.