A method for manufacturing a high pressure tank includes a step of forming a reinforcing pipe portion including a straight pipe portion and reduced-diameter portions, a step of forming reinforcing dome portions, a step of forming a joined body by joining the reinforcing pipe portion and the reinforcing dome portions, and a step of forming an outer helical layer on an outer face of the joined body. The reinforcing dome portions are disposed with opening ends of the reinforcing dome portions positioned on outer faces of the reduced-diameter portions of the reinforcing pipe portion.