An apparatus and method are provided for producing a hollow molded product having a thick part free from a thin part or defect. According to the method, a mold is designed to have a cavity for a hollow molded product having a bent portion such as an assist grip. This mold is provided with dams mounted in a gas passage around an in-course in the bent forming portion. When injecting fluid into the already injected resin in the cavity, the dams are so designed to function in guiding the resin to flow between the dam and an out-course of the cavity. By this method, sufficient amount of the resin remains around the in-course so as to form a thick part of the hollow molded product.