PROBLEM TO BE SOLVED: To reduce the conversion fore by maintaining the reaction force at a working point nearly constant or changing so as to be gradually increased or gradually reduced in relation to a displacement of the working point.
SOLUTION: A spring 13f is arranged in a cylinder part 13a so that the repulsive force is applied to the cylinder part 13a. The pressure from the spring 13f can be variable for adjustment and the support pressure of a roller 14 can be variable for adjustment. A tongue rail T to be adhered to a basic rail R is placed on a floor plate P. The roller 14 is set higher than a top surface of the floor plate P. When the tongue rail R starts the conversion and separates from the tight fitting position, a bottom part of the tongue rail T pushes the roller 14 downward. The push-down force is transmitted from a roller shaft 12a to a crank 12, and the reaction force at the working point is maintained nearly constant or gradually increased or gradually reduced in relation to a displaced variable of the working point. With this structure, fine adjustment of the roller height is unnecessary, and the labor of maintenance is reduced.