To provide a gas discharge preventing device capable of surely preventing gas discharge with a simple structure and high durability.

This gas discharge preventing device comprises a main body case 10 connected with a valve 50 of a gas storing container, a movable body 20 slidably mounted in the main body case 10, and a stopper 30 for fixing the movable body 20 to a position to open the valve 50 by the movable body 20, and the movable body 20 fixed by the stopper 30 is released when more than prescribed tensile force is added to the movable body so that the movable body 20 is slid to open the valve 50.