An automatically latching lock mechanism for locking a door, which comprises a latch mechanism mounted in the body (11) of a door and a pull bar (8) connected thereto. The latch mechanism comprises a latch (13) turnable on a shaft, which preferably is a flat-shaped member having an asymmetric periphery and turnable around the shaft and provided with at least two radially outwardly projecting protrusions (1, 2) which form a locking latch (2) which in the locking position of the lock mechanism protrudes from the body (11) of the door extending into a recess or the like formed in the door frame (10), and a detecting latch (1) which in the unlocked position of the lock mechanism protrudes from the body of the door and when closing the door hits the door frame thereby turning the latch so that the locking latch (2) is pushed out from the body of the door.
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