The present invention provides a method of manufacturing a pipe with a branch, which utilizes the conventional pipe molding method using a floating core, and while taking advantage of the pipe molding method, various types of pipes with a branch can be integrally molded with a synthetic resin with high efficiency. The method comprises injecting a molten resin into a main cavity 4, which has on its one end a pressure port 2 provided with a floating core 1 and on its other end a first outlet 3, and a branch cavity 5, which communicates with the main cavity 4 and has on its end a second outlet 6 allowed to open and close, pressure-injecting a pressurized fluid through the pressure port 2 after the injection of the molten resin, and moving the floating core 1 to the first outlet 3 side, and, at the same time, extruding the molten resin from the first outlet 3 to form a main pipe hollow, and opening the second outlet 6 after the main pipe hollow reaches a portion at which the branch cavity 5 communicates with the main cavity 4, and extruding the molten resin by the pressurized fluid from the second outlet 6 to form the branch pipe hollow.