PURPOSE: To prevent occurrence of accidents with entering by anyone into a hazardous space made impossible by a method wherein a sensor monitoring the atmosphere in an enclosed space is provided and a door is made unopenable when a hazardous condition exists in the space.

CONSTITUTION: When a signal verifying the oxygen density in a specified enclosed space being lower than a safety level is received from a sensor 31, a controller transmits a signal to a solenoid 21 and makes a plunger 22 project. The solenoid 21 is a two-way locking solenoid which is formed, penetrating through a door 2, and with one of two coils electrified, the plunger 22 is projected and with the other coil electrified, the plunger 22 is returned to its original position. As the plunger 22 is projected, the plunger 22 reaches a acceptance port 17 of a stop mount 14. The plunger 22 retards turning of a swing stop 12 at this position and raises a latch handle 54 upward, making it incapable of being detached from a catch 11.