Method and apparatus for manufacturing a synthetic resin-made hollow member incorporating an intermediate element are provided which enable operations of molding half bodies, abutting and joining the half bodies together, and molding an intermediate element in a series of steps, and which eliminate the necessity of such a manual work as may otherwise be required in incorporating the intermediate element into the half bodies. A pair of rotary injection molding dies are used which can be opened and closed relative to each other and are rotatable relative to each other over an angle of 60° for each turn, each die having a half body molding section including one male molding portion and two female molding portions in the direction of rotation for each rotational run over an angle of 120°, each die also having an intermediate element molding portion provided between specified half body molding portions in the direction of each rotational run over an angle of 120°. In each two rotational runs of the molding dies, a series of steps is carried out including first injection for molding a pair of half bodies and an intermediate element, molded product setting for matching two half bodies with the intermediate element set in place and abutting the two half bodies against each other, and second injection for injecting melted resin onto the abutting portions and joining the two half bodies, whereby a hollow member with the intermediate element set between the two half bodies can be obtained.