A gas injecting device for an injection molding machine is equipped at one side of a cylinder of the injection molding machine, and includes a pin inserted into a gas injection hole of the gas injecting device. The pin has flat sections formed around an outer lower portion of the pin to define gaps between the flat sections and an inner surface of the gas injection hole, such that a gas is injected uniformly through the gaps, thereby allowing the gas and a resin to be easily mixed while preventing the resin from flowing backwards through the gas injecting device. An amount of gas injected to a barrel of the injection molding machine is accurately controlled according to a pressure difference between a gas line and the barrel, and time data, allowing an article to be injection molded according to a kind of resin or characteristics of the article.