An injection device of light metal injection molding machine and an injection control method thereof are provided, in which a melt in a supply unit is supplied into an injection unit through a communication passage, a plunger of the injection unit is retracted to measure the melt, the communication passage is closed, and the plunger is advanced to inject the melt into a mold device through an injection nozzle of the injection unit. After the injection and before the measurement, the plunger is advanced under a pressure at which the melt does not come out from the injection nozzle to make the melt in the injection unit flow back into the supply unit through the opened communication passage.