To provide an emergent opening system of a door preventing injury of persons and damage of instruments by not manually opening but also automatically opening even if an instantaneous atmospheric pressure difference arises between the inside/outside of a room due to an explosion or the like, in a lock relief system in which usually one enters indoors after unlocking a door lock from the outside and goes out from the inside by a handle.

The present system is so constituted that a sensor door is automatically opened owing to a pressure difference between the inside/outside of a door, in the case of outdoor explosion or the like to instantaneously make even the atmospheric pressure between the inside/outside of the door in order to protect persons and instruments, by retrofitting a locking unit in which a locking bolt is fitted, a connection structure connected through a locking bolt and a connecting means pulling inwards the door, and a sensor door connectedly installed in the structure through a fixing means by using a conventional lock relief system in which one usually enters indoors from the outside by unlocking the door and goes out outdoors from the inside by pulling the locking bolt by use of a handle.