PURPOSE: To increase body strength with almost no increase in body weight by connecting the rear ends of side members to each other with a cross member wherein the side members are obliquely disposed in such a way that the rear and front ends of them are connected to each tunnel reinforcement and each front side frame respectively.

CONSTITUTION: Front side frames 21 withstand backward load, and backward load caught by the front side frames 21 is dispersed into tunnel reinforcements 51 via side frames 7 which are disposed obliquely. In addition, since a spare tire 9 is disposed so as to be fixed onto a cross member 10 in such a way as to be held between paired side frames 7 obliquely disposed, the spare tire 9 will never be moved upward while it is kept between the side frames 7. Thereby this enables load to be dispersed because the spare tire 9 pushes the tunnel reinforcements 51 back while it is widening a space between the side frames 7.