To provide a method for simply and stably molding a thick-walled molded article not having a flaw such as voids, a sink or the like and good in appearance with good productivity while shortening a molding cycle.

In the method for manufacturing the thick-walled molded article by injection-molding a thermoplastic resin using a hot runner mold in such a state that the resin in the hot runner gate part is held to a molten state, the temperature of the mold at the time of injection of the molten resin in the mold is heated to a temperature range from the deflection temperature under load to the melting point + 20°C and, simultaneously with or 0-10 sec before the start of the injection and charging of the molten resin in the mold, a heating means is changed over to the cooling means to cool the mold to a temperature lower than that of the mold at the time of heating by -10°C or lower and, in order to hold the resin of the gate part to a molten state, the gate tip, body and manifold of the hot runner mold are held to a heated state always or for a specific period.