A method of manufacturing a liquid ejection head, and the method includes a process of providing a first photosensitive resin layer containing a photodegradable positive photosensitive resin and serving as a mold material of the channel on a substrate, a process of providing a gas barrier layer having a film density of 1 g/cm3 or more on the first photosensitive resin layer, a process of subjecting the first photosensitive resin layer and the gas barrier layer to pattern exposure, and then performing development to thereby form the mold material of a channel and also removing the gas barrier layer before or simultaneously with the development, a process of providing a second photosensitive resin layer on the mold material and the substrate, a process of subjecting the second photosensitive resin layer to pattern exposure, and then performing development, and a process of removing the mold material of the channel.