To secure sealing performance by eliminating a gasket in the mutual connection between a pair of flanges, and reduce a parts count.

An uneven part 13 having myriad fine unevennesses by applying surface reformation treatment such as shot-blast treatment to a region annularly coupled on an outer peripheral side of an opening is formed in a fastening surface of flanges 11 in a pair of members 1A, 1B with a flange made from a metallic material or the like. The flanges 11 are mutually fastened in an axial direction with a bolt and nut. By so doing, the uneven parts 13 of respective flanges are crushed by a mated uneven part 13 to obtain a high sealing effect. As a result, a gasket can be eliminated.