PURPOSE: To provide a high strength to the connecting surface of a member, particularly when a flat first member and a second member having a curved surface in the corner part of the inner wall surface are mutually fastened in the curved surface part, absorb the manufacturing common difference, and perform the work and manufacture with a dimensional precision as designed.

CONSTITUTION: A highly strong aluminium block member 14 is arranged on the curved surface part 12b of the inner wall surface 12a of a second member 12, and this block member 14 has a curved surface part 15 to be touched and engaged in conformation to the curved surface form of the corner part of the second member 12. It has a through-hole 16 formed in a position opposed to the through-holes 11, 13 of the first member 10 and the second member 12, and a plate nut 18 to be screwed to the top end part of a screw member 17 is integrally mounted on the outside surface of the block member 14.