PURPOSE: To stably hold a mobile member by providing an energizing means which movaly holds a second shaft by a long hole provided on a fixed base and energizes it in the long hole in the direction where a coiled spring is expanded and gradually increasing the torque according as the mobile member approaches the vicinity of the end point of rotation or this end point.

CONSTITUTION: An energizing means 64 is provided which movably holds the second shaft 56 as the fulcrum of a coiled spring 62 by a long hole 60 provided on the fixed base and energizes the second shaft 56 in the long hole 60 in the direction where the coiled spring 62 is expanded, and a guide groove of a grooved plate of the mobile member is formed into such shape that the mobile member is guided in a direction to gradually increase the torque according as the mobile member approaches the vicinity of the end point of rotation. Thus, the upward torque acting upon the mobile member is gradually increased by a torque generating means according as the mobile member approaches the end point of rotation from its vicinity, and the mobile member is stably held though it is at the end point of rotation or in its vicinity.