PURPOSE: To restrain a latch bolt surely in an unlocking position with a simple operation while reducing substantially a risk of dishonest unlocking by an unauthiruzed person.

CONSTITUTION: A latch bolt 6 received in a case 2 is slidably urged in the direction of engaging the fixed frame body 4 side lock hole 5 by a spring 3. A lock lever 8 received in a slit groove of the latch bolt 6 is supported by the latch bolt 6 to rotatably go in and out of the slit groove and urged by the spring 3 in the direction of plunging the tip part into the slit groove. A case 2 is provided in the front face part with a key hole 40 along the direction of moving an operating part 11 of the lock lever 8. When the latch bolt 6 is slid backward to the unlocking position by pushing the operating part 11 of the lock lever 8 with an operating projection 39 of a key 38, the tip end face 8a of the lock lever 3 is adapted to abut against the hole edge 5a of the lock hole 5.