A method of manufacturing a liquid ejection head including a first step of having a molten resin becoming a first portion flow in between a first position in a first mold and a first position in a second mold and the molten resin becoming a second portion flow in between a second position in the first mold and a second position in the second mold; a second step of relatively moving the first and second molds, after opening the molds; a third step of closing the molds; a fourth step of joining the first and second portions, a fifth step of taking out the first and second portions and the sealing portion from the molds and mounting a recording element substrate; and a step of forming a space portion between a liquid supply path surface and a mounting surface.