Abstract: An injection molded product including a plurality of internal cavities is produced by, after having injected a molten resin material into the molding cavity, injecting an inert gas with a gas pressure gradually increasing into the resin material through gas injection nozzles, which are in contact with the resin material to form the respective internal cavities inside the molten resin material. After removing the gas injection nozzles from the resin material for discharging of the inert gas directly from the resin material, any one of the gas injection nozzles, whose temperature during injection of the inert gas was lower than a predetermined temperature level, is judged to be clogged and the gas injection nozzle is cleaned by being blown out with high pressure gas.