To facilitate the positioning work in a rotation transmitting mechanism capable of positioning in the peripheral direction.

A first shaft 10 and a second shaft 20 are combined by a sleeve form of coupling case 30. The second shaft 20 and the coupling case 30 are combined through helical gears 21 and 34, in order to convert the linear movement in the axial direction of the coupling case 30 into the rotating movement of the second shaft 20. The coupling case 30 has a thin wall part 32 at the end of the second shaft side, and a fastening member 40 is installed to the thin wall part 32 from the outer side. The fastening member 40 has a clearance 41 formed in the circumferential direction along the inner peripheral surface, and by adding a fluid pressure to the clearance 41, a part at the inner surface side from the clearance 41 is swelled and deformed to the inner surface side, so as to fasten the thin wall part 32. In the condition no fluid pressure is applied to the clearance 41, the coupling case 30 is movable in the axial direction.