To forcibly open a door by adding an auxiliary force in an opening direction to the door, even when pressure inside a room becomes high and the high pressure causes biting in a lock.
In a door opening assist device 36, a fan 38 rotated by the difference between indoor and outdoor pressures is provided in a pressure release port 37 formed in the door 21, and electric power is generated by a power generator 40. The electric power is transmitted to a drive motor provided on the door 21, whereby a door opening assist mechanism 45 is actuated to impart the auxiliary force in the door opening direction. In the door opening assist mechanism 45, racks are formed in an operating shaft 47 rotatable around the base shaft 46 installed on a wall surface 41. The rotating power of the drive motor is transmitted to the racks by a pinion to impart a tensile force to the operating shaft 47. A battery 54 for storing electric power, when a timing lag occurs at opening the door, is connected inparallel to the drive motor by wiring 42. When the door 21 is opened, the operating shaft 47 is relatively pulled by the rotating power of the drive motor receiving the electric power from the battery 54 to impart the auxiliary force in the door opening direction to the door 21.