PURPOSE: To enhance the strength and stiffness of a car body side part by connecting the side edge part of a cross member to a side sill inner panel with a thick plate member arranged above a stepped part.

CONSTITUTION: The side edge part 6a of a cross member 6 which is arranged over a floor panel 5 while a closed cross part is formed in a space interspaced the floor panel 5, is connected to a side sill upper panel 3 which forms a side sill 1 in a shape of a closed cross section, with a thick plate member 7 arranged above a stepped part 5b. By this constitution, the circumference of the joint of the side edge part 5a of the floor panel, which forms a car body side part and a side sill 1, is thereby enhanced in strength and stiffness. This configuration accordingly enables the thickness of the cross member 6 to be reduced to the extent of strength and stiffness required, thereby making it possible to lighten the cross member 6, so that a car body weight is reduced.