PURPOSE: To enlarge a filing ring open distance at a small open angle by a method wherein a movable member is pivotally attached to a lower end of a movable substrate by always energizing in a lock closing direction, and a fixed lock member of a specific shape which pivotally moves the movable lock member by acting thereon and is engaged with the movable lock member is provided to a lower end of a fixed substrate.

CONSTITUTION: A hole having a straight line part 45 is provided on a fixed substrate side 1, a shaft 40 having a leaf spring 44 of a movable lock member 4 on a movable substrate 2 side is fitted thereto, and bias force in a lock closing direction is always energized. A detent part 46 is, formed to the movable block member 4. Besides, a fixed lock member 3 is formed on a fixed substrate 1 side, a detent face 47 opposed to the detent part 46 is formed, and besides a tapered face 48 is formed. Then, the movable lock member 4 is opened by making cam operation act on the movable lock member 4 with the tapered face 48 by closing action of the movable substrate 2. Thereafter, the detent part 46 is engaged with the detent face 47.