There is difference between coefficients of thermal expansion of the first member and the second member. A mismatch occurs between the walls of the first member and the second member when the temperature is changed significantly. Stress generated by this mismatch is distributed among and received by respective fitting sections of respective concave portions and respective convex portions. Because these fitting sections exist above and below the joining member, stress generated by the mismatch is distributed and received by the fitting sections before the stress reaches the joining member. Therefore, only a little stress acts on the joining member.