A method for manufacturing an outer conductor of a higher-order-mode coupler for a superconducting acceleration cavity. The outer conductor includes a cylindrical main body that is open at one end surface thereof, a port formed in a side of the main body so as to penetrate therethrough, and a protruding part formed outside another end surface of the main body. The method includes a deep drawing step of deep-drawing a metal plate to form the main body, a port-forming step of flanging the thus-formed main body to form the port, and a first machining step of machining the main body to adjust an outer shape thereof.