A method of manufacturing a fuel tank includes: passing a hollow molten resin projection, that projects-out in a wall-thickness direction and is formed at a molten resin sheet that becomes a tank structural member structuring a tank main body, through a mounting hole of a part-to-be-mounted that is placed on the molten resin sheet; and pressurizing the molten resin projection from an inner side, and causing at least a portion of the molten resin projection to jut-out to a peripheral portion of the mounting hole of the part-to-be-mounted.