To provide a device for preventing the opening of a door, capable of preventing the door from being opened if it is unlocked illegally by picking or the like, so as to enhance security.

The device 1, for preventing the opening of a door by abutting against the open-side peripheral edge of the door to regulate the opening of the door, is provided with an attaching-rod insertion plate A secured to a door frame and having an insertion cylinder 7 positioned on the outside of the track of the opening movement of the door; an attaching rod B inserted into the insertion cylinder 7 of the plate A, with the rear end 15a of the rod positioned on the inside of the track of the opening movement of the door to abut against the open-side peripheral edge of the door to regulate the opening of the door; and a lock mechanism member C locked to the front end of the rod B inserted in the cylinder 7 to regulate removal of the rod B from the cylinder 7, with the lock mechanism member being unlocked only by dialing operation using, e.g. a plurality of unlocking operation rings 23a to 23d.