To provide a lock mechanism to lock/unlock by engaging/disengaging a locking lever with a step part of a latch head, so as never to be unlocked by an earthquake or vibration by any chance.

A first support shaft 7 is provided near a latch bolt, which serves also as a dead bolt, and a tip end of the locking lever 8 whose other end is pivotably supported is energized to a direction for engaging with the step part of the latch head 4. Therein, near the first support shaft, a second support shaft 22 is provided at the opposite side from the latch bolt 1 with respect to the first support shaft 7. A braking lever 21 is rotatably supported to the second support shaft 22 on a plane surface parallel to a side plate 2a of the lock case and its front end is energized to a direction to be disengaged from the latch bolt 1. Also, a shape and a dimension of the braking lever is determined in such a way that a center of gravity of the braking lever is positioned in its rear end part.