To easily discriminate a recycling time by making the skin layer of a molded product thin and forming cells in a thick wall portion uniformly in the same size or forming the cells on the side of the skin layer small or relaxing an external impact without using a different kind of a material.

At the injection molding of a molten resin mixed with a blowing agent, the vol. of a cavity 3 in a mold is set to a final objective vol. or less and the surface temp. of the mold is heated to the Tg of the resin or higher and the mold is filled with the resin 4 and, subsequently, the mold is opened so that the vol. of the cavity 3 becomes the final objective vol. and cooled. By this constitution, the skin layers 6 of a molded product are formed thinly and hollow parts 5 are uniformly formed in the thick wall portion between the skin layers. By opening the mold stepwise, the diameter of the hollow parts 5 is made large at the center portion between the skin layers 6 and made small at the portions near to the skin layers 6.