To suppress incorrect follow-up of a cam follower with respect to a tubular cam while enhancing durability through a simple structure by constructing means for urging one cam follower toward the tubular cam side of a leaf spring.

Under a state where a holder 43 is fixed with upper and lower cam followers 44a, 44b and set to a tubular rim cam 46, the lower cam follower 44b is urged, by a leaf spring 47, toward the tubular rim cam 46 which is thereby clamped by the upper and lower rim cam followers 44a, 44b. When the upper and lower rim cam followers 44a, 44b come into rolling contact with the tubular rim cam 46, fluctuation in the thickness of the tubular rim cam 46 itself and at the inclining part (curved part) thereof is absorbed by the leaf spring 47 and a mounting head moves up and down accurately and smoothly. Since the leaf spring 47 extends not in the tangential direction of the tubular rim cam 46 but in the axial direction thereof, the lower cam follower 44b inclines upon flexure of the leaf spring 47.