To provide a device for blocking gas discharge composed of simple structure, which accumulates power generated by a power spring, releases the power accumulated in the power spring when there occurs an unanticipated situation, and promptly close a main cock of a gas cylinder in the state of gas being discharged, by the power.

The main cock automatic closing device is characterized by comprising: a means for accumulating the power of the power spring by the forward rotation of a handle, and additionally, a means for turning a main-cock installed cylinder of the gas cylinder by using a plurality of gears which engagedly rotate in conjunction with the forward rotation of the handle; and a means which has a gear engaged with one of the gears for turning the main-cock installed cylinder of the gas cylinder, receives the transmission of a stopper function by a stopper device by using the gear, holds the power accumulated by the forward rotation of the handle and the turning state of the main-cock installed cylinder of the gas cylinder, and releases the stopper function by stopping air supply to the stopper device.