Raw material of less than 10% of the one injection amount is heated at a time to a high temperature just before injected into the mold cavity by an intermittent heater and is injected continuously into the cavity keeping that temperature. Heat capacity of the heater is small to obtain quick response and to give no heat effect to the raw material to be injected for the next product. The device of this invention comprises a device to inject a predetermined amount of raw material for one injection intermittently, a flow path for the raw material which connects said device and the mold, and electric resistant heater in the flow path to generate Joule heat and to transfer the generated heat to the raw material flowing through said flow path.