A coupling mechanism for a rotary fluid cylinder which comprises a rotary member and a fluid feeding member concentrically mounted on said rotary member, at least one fluid passage being formed for supplying fluid from the fluid feeding member to the rotary member even while the rotary member is rotating. The coupling mechanism comprises a pair of rolling bearings interposed between the fluid feeding member and the rotary member at the respective extreme ends of the fluid feeding member so as to allow relative rotation of the rotary member with respect to the fluid feeding member. A biasing member for applying a predetermined pre-load to the respective rolling bearing is provided. The applied pre-load acts on the rolling bearings in such a manner that the rolling bearings are biased to move away from each other in their axial direction.
