PURPOSE: To provide an automatic lock which is suitable for applying to the door to partition the staff room of a rolling stock from the cabin.

CONSTITUTION: An automatic lock device concerned is so structured that a key must be used to unlock from the cabin side, wherein a slider 3 with which a latch bolt 2 working also as a dead bolt is connected through a spring 7, is drawn into a casing 1 against the action of a main spring 10 using an indoor unlocking cam 11 to be rotated by an indoor grip and an out door unlocking cam 14 turned by an outdoor grip. The outdoor can 14 is always fixed by a locking can 18, and key is used from the outdoor side in order to release the outdoor cam 14 fixed by the locking cam 18 while, for example, a some-turn is used from the indoor side for the same purpose. If necessary, a fixing lever is installed to provisionally fix the locking cam 18 in the non-fix position so that locking takes place automatically as long as the door is shut after each door opening.