An injection molding system comprising: a mold clamping device for opening and closing a mold and an injection device for injecting a molding material into a cavity of said mold; a heating medium supply device for supplying a heating medium to a heat medium passage (30, 31) formed in said mold for heating said cavity; a cooling medium supply device for supplying a cooling medium to said heat medium passage (30, 31) for cooling said cavity; and a control unit for controlling supply of said heating medium and said cooling medium by said heating medium supply device and said cooling medium supply device, which is configured to supply said heating medium to said heat medium passage (30, 31) to heat said cavity when said molding material is injected from said injection device into said cavity, and configured to supply said cooling medium to said heat medium passage (30, 31) in conjunction with an injection step of said molding material to cool said cavity, wherein said control unit is configured to start the injection of said molding material or stop heating of said cavity during the injection before a temperature of said cavity reaches a predetermined temperature range so that the temperature of said cavity does not exceed the predetermined temperature range during the injection of said molding material due to a delay of heat transfer, and said control unit is configured to then maintain the temperature of said cavity within said predetermined temperature range after the temperature of said cavity reaches said predetermined temperature range and until said cooling medium is supplied to start cooling of said cavity, wherein the control unit is configured to maintain the temperature of said cavity within said predetermined temperature range by alternating between natural heat dissipation by stopping the supply of the heating medium and reheating with the supply of the heating medium. a mold clamping device for opening and closing a mold and an injection device for injecting a molding material into a cavity of said mold; a heating medium supply device for supplying a heating medium to a heat medium passage (30, 31) formed in said mold for heating said cavity; a cooling medium supply device for supplying a cooling medium to said heat medium passage (30, 31) for cooling said cavity; and a control unit for controlling supply of said heating medium and said cooling medium by said heating medium supply device and said cooling medium supply device, which is configured to supply said heating medium to said heat medium passage (30, 31) to heat said cavity when said molding material is injected from said injection device into said cavity, and configured to supply said cooling medium to said heat medium passage (30, 31) in conjunction with an injection step of said molding material to cool said cavity, wherein said control unit is configured to start the injection of said molding material or stop heating of said cavity during the injection before a temperature of said cavity reaches a predetermined temperature range so that the temperature of said cavity does not exceed the predetermined temperature range during the injection of said molding material due to a delay of heat transfer, and said control unit is configured to then maintain the temperature of said cavity within said predetermined temperature range after the temperature of said cavity reaches said predetermined temperature range and until said cooling medium is supplied to start cooling of said cavity, wherein the control unit is configured to maintain the temperature of said cavity within said predetermined temperature range by alternating between natural heat dissipation by stopping the supply of the heating medium and reheating with the supply of the heating medium.