To provide an injection port of an automatic fire extinguishing device preventing intrusion of steam of solvent, fat and the like into the injection port, and eliminating malfunction due to thermal expansion of air in the injection port and detachment of components when releasing fire extinguishant.

This injection port of the automatic fire extinguishing device, when a sensor part senses a fire, automatically opening a fire extinguishant container and releasing the fire extinguishant from the injection port to a fire extinguishing target area is provided with a piston valve at the tip of the injection port for regularly closing the injection port and capable of opening the injection port by a pressure caused by opening the fire extinguishant. Further, the circumference of the piston valve is provided with a sealing part for preventing the intrusion of gas in ambient environment into the injection port, and the circumference of the piston valve inside the injection port of the sealing part has a retaining part for preventing the operation of the piston valve by the expansion of the gas in the injection port. Preferably, a stopper part is provided for preventing the falling of the piston valve from the injection port when opening the injection port.