In the present invention, in a piercing punch and a burring punch which are shaft members having a first axis line as the center axis lines and are provided so as to be movable along the first axis line, first, after prepared holes having the first axis line as the center axis lines with respect to the first wall portion and the second wall portion are formed using the piercing punch, burring holes having the first axis line as the center axis lines with respect to the first wall portion and the second wall portion are formed using the burring punch.