A method for manufacturing a resin pipe joint with a joint body includes the following steps. Preparing a mold and a supply device configured to supply molten resin. The mold includes a first mold for an outer shape of the joint body and a second mold for an inner shape of the joint body. Combining the second mold with the first mold to form a cavity, into which the molten resin is injectable, inside the mold. Supplying the molten resin to the cavity in the mold. Solidifying the molten resin in the cavity after stop of the supply of the molten resin from the supply device. The second mold has a gap that connects the cavity to a discharge space outside the cavity and allows gas to flow.