Disclosed is a joining method of members, which includes the steps of: preparing a first member in which a first hole portion having a polygonal cross-sectional shape is formed, and a second member having a hollow shape, wherein the second member has a cross-sectional shape corresponding to the polygonal cross-sectional shape of the first hole portion, the second member including a plurality of straight line portions extending linearly and corner portions positioned between the adjacent two straight line portions in the cross-sectional shape; inserting the second member into the first hole portion of the first member; and caulking and joining the second member to the first member by enlarging and deforming the corner portions.