Abstract: A gas vent system of a mold improves timing of closing the gas vent passageway, in particular in die castings or in moldings. The gas vent passageway is formed on mating surfaces of a pair of mold halves and communicating a mold cavity with the outside of a mold when the mold is closed. The gas vent valve is mounted on one of the mold halves and disposed intersecting perpendicularly to the gas vent passageway and is provided with a concave portion on its end surface receiving the pressure from the molten pressure charged. The other mold half is provided with a projection which can be fitted tightly in the concave portion of the gas vent valve, and the projection is provided with part of the gas vent passageway so as to direct a flow of the molten material passed through the gas vent passageway from the mold cavity toward the concave portion of the gas vent valve. The gas vent valve is operated to close the gas vent passageway in response to a pressure from the molten material forced into the mold cavity.