PURPOSE: To obtain a hollow panel having reinforcing ribs therein by advancing a slide core fitted in one mold and equipped with projections into the cavity. forming a small narrow part between the projections and another mold for injecting resin, and withdrawing the slide core while supplying high pressure gas.

CONSTITUTION: The stationary mold 2 has a slide core 7 fitted therein which is impulsively movable toward a cavity C defined by itself and a movable mold 3. The slide core 7 is provided with a plurality of projections at a predetermined distance and when situated in the withdrawal position, the smooth surface 7a between projections is caused to be in flush with the cavity surface of a mold main body 6. At an advancing position, it runs into the cavity C for defining a small narrow part between the upper surface of the projections and the smooth surface 3a of the movable mold 3. At the advancing position, when molten resin is infected and high pressure gas is supplied, a hollow part is formed. As the supply of high pressure gas is supplied and the slide core 7 is withdrawn, the volume of the hollow part increases and, as a result, the wall thickness of molten resin in the small narrow part comes to be thin and thus be reinforcing ribs.