PURPOSE: To smoothen the raising operation of a carriage as well as to convert the force of spring into a holding force with good efficiency by maximizing the spring force under the condition that the carriage is held.

CONSTITUTION: A clamp 7 is designed to be turned around a supporting point M as the center, and is given a counterclockwise turning force by a spring 6. By the force, the carriage-supporting projection 5 is pressed on a lock base plate 4A. The force (f1) of pushing the projection 5 by the clamp 7 is obtained from the spring 6. By providing the supporting point 9 of the spring 6 at the position where f1>f2 results, however, the force of pushing the projection 5 is maximized. By such a constitution, since the moment of an arm (l1) and the force (f1) acts on the projection 5, the carriage can be held with small forces.