A method for manufacturing a hollow part with a branch, including a resin filling step of filling a mold, the mold having a main cavity in which a hollowing piece is placed, and a branch cavity to communicate with the main cavity, in which a slide shaft is inserted to advance and withdraw, and that forms a branch pipe portion in an annular space between the branch cavity and the slide shaft, a step of forming a hollow main pipe portion by moving the hollowing piece to force excess resin out of the main cavity, and a shaft removing step of withdrawing the slide shaft to form a branch hollow portion, the hollow part with a branch being made of a resin and having the branch pipe portion integrally connected to the main pipe portion such that the main hollow portion and the branch hollow portion communicate with each other.