PURPOSE: To form a recessed part becoming a handle to the outer housing of a household electric appliance or OA machinery by a hollow molding method.

CONSTITUTION: An air mold and a cavity mold are used and a resin 1 is sealed in the space between those molds and gas 2 is injected into the thick-walled part A of the resin. When the gas 2 is increased to hold pressure until the gas 2 comes into contact with the cavity mold, the recessed part 3 opened toward the cavity mold is formed.