Method and apparatus of manufacturing a synthetic resin tubular member having differently oriented inlet tube and outlet tube portions uses a pair of displaceable molding dies. One of the dies is rotatable relative to the other die. The molding dies have a core portion fitted to the tube end of one of the inlet and outlet tube portions and is slideable in a direction different from the direction in which the dies open and close. The method includes the steps of core insertion, die clamping, injection, die opening, core removal, ejection, and die rotation. Each time the molding die is rotated one step, a primary molding step, in which two molding halves of the tubular member are formed, and a secondary molding operation, in which the two molding halves are joined, are formed, so that a finished tubular member is obtained for each rotational movement of the molding dies. A melted resin is filled into an interior passage defined along a peripheral edge of one of the dies after the molding dies are brought into abutment with each other to join the two halves. The finished product can be a synthetic resin manifold. The manifold has a plurality of outlet tube portions branching from an inlet tube portion. One of the two halves has both the inlet and out tube portions. That is, the inlet and outlet portions are monolithic to provide seamless inlet and outlet.