PURPOSE: To automatically open a door at earthquake time not less than specific seismic intensity by expanding an iron ball falling shock in an earthquake sensing mechanism by the principle of the lever, pressing a dead bolt and a latch, and simultaneously releasing locking of a door releasing mechanism.

CONSTITUTION: A door lock releasing mechanism 8, a door releasing actuating mechanism 9, a door lock releasing actuating mechanism 10 and an earthquake sensing mechanism 11 are internally arranged in a door frame 2. One end of a door releasing mechanism 7 by bending an elastic member in an almost U shape is fixed to an upper part of the door frame 2, and the other end is locked on the upper end 9a of the door releasing actuating mechanism 9. An iron ball 16 in a holding part of the sensing mechanism 11 falls by an earthquake not less than specific seismic intensity, and collides with a lower part 10b of the door lock actuating mechanism 10, and the upper end 10a is moved in the leftward direction by releasing locking of a bolt pressing block 13, and presses a dead bolt 5 and a latch bolt. At the same time, a projecting part 13a of the bolt pressing block 13 presses a lower end part 9b of the door releading actuating mechanism 9 in the leftward direction, and an upper end part (a) moves in the rightward direction, and slips out of the other end of the door releasing mechanism 7, and opens a door 1.