PURPOSE: To let a fuel tank and the like be disposed with no car body weight increased by letting the front end of a second side member be connected to the rear end of a first side member, and concurrently letting the rear end of the second side member be connected to the front end of a third side member.

CONSTITUTION: A first side member 20, a second side member 21 and a third side member 22 are continuously disposed reasonably to a rear floor panel 10 which is furnished with the difference in height in a form of a first panel section 13, a second panel section 14 and a third panel section 15. Since a fuel tank 28 is disposed below the first panel section 13 of the rear floor panel 10, that is, the outside of a cabin, there is no need for providing a fire prevention wall around the fuel tank 28, therefore, cost can thereby be reduced by that much.