An injection device of a light metal injection molding machine, comprising: a melting unit, comprising a melting cylinder, wherein an opening is formed on a rear end surface of the melting cylinder in order to sequentially supply a light metal material in a cylindrical short rod shape into the melting cylinder configured in a manner of being pushed along a cylinder hole is heated and melted into molten metal in the melting cylinder; an injection unit, comprising an injection cylinder and a plunger, wherein the molten metal in the melting cylinder can be supplied into the injection cylinder by free fall due to own weight of the molten metal in the melting cylinder; and the molten metal stored in the injection cylinder is injected by the plunger inserted so as to be capable of moving back and forth in the injection cylinder; a connection member that connects the melting unit and the injection unit, in which a communication path communicating the inside of the melting cylinder with the inside of the injection cylinder is formed; and a molten metal pot in which a molten metal supply/discharge port is formed supplying and discharging the molten metal, wherein the the molten metal pot is connected to the melting cylinder in a manner that at a location in the melting cylinder except for one facing an opening of the communication path on the melting cylinder side, the molten metal supply/discharge port is opened so as to communicate with the inside of the melting cylinder; and the injection device further comprises a backflow prevention device opening and closing the communication path, and a liquid surface level detection device that detects a height of a liquid surface of the molten metal stored in the molten metal pot, an inner diameter of the molten metal supply/discharge port is formed to be larger than an inner diameter of the communication path and smaller than an inner diameter of the molten metal pot. a melting unit, comprising a melting cylinder, wherein an opening is formed on a rear end surface of the melting cylinder in order to sequentially supply a light metal material in a cylindrical short rod shape into the melting cylinder configured in a manner of being pushed along a cylinder hole is heated and melted into molten metal in the melting cylinder; an injection unit, comprising an injection cylinder and a plunger, wherein the molten metal in the melting cylinder can be supplied into the injection cylinder by free fall due to own weight of the molten metal in the melting cylinder; and the molten metal stored in the injection cylinder is injected by the plunger inserted so as to be capable of moving back and forth in the injection cylinder; a connection member that connects the melting unit and the injection unit, in which a communication path communicating the inside of the melting cylinder with the inside of the injection cylinder is formed; and a molten metal pot in which a molten metal supply/discharge port is formed supplying and discharging the molten metal, wherein the the molten metal pot is connected to the melting cylinder in a manner that at a location in the melting cylinder except for one facing an opening of the communication path on the melting cylinder side, the molten metal supply/discharge port is opened so as to communicate with the inside of the melting cylinder; and the injection device further comprises a backflow prevention device opening and closing the communication path, and a liquid surface level detection device that detects a height of a liquid surface of the molten metal stored in the molten metal pot, an inner diameter of the molten metal supply/discharge port is formed to be larger than an inner diameter of the communication path and smaller than an inner diameter of the molten metal pot.