A molding apparatus for producing a synthetic resin molded product including a forming mold with a molding cavity, a gate communicating with the cavity, and an overflow chamber communicating with the cavity through a flow-out passage, a gas blow-in mechanism for blowing gas into molten synthetic resin which has been injected into the cavity through the gate so as to form a void in the molten synthetic resin in the cavity to mold a hollow product and a flow-out passage opening and closing mechanism for closing the flow-out passage until the molten resin fills the molding cavity and opening the flow-out passage when the gas blow-in mechanism is in operation.