PURPOSE: To prevent a stess concentration, to enhance the stiffness and to facilitate getting in and out of occupant by arranging a side sill to be increasingly higher toward the rear.

CONSTITUTION: The side sill 4 is, as shown in the drawing, arranged lower toward the front to facilitate for occupants to get in and out, and increasingly higher toward the rear. Further, the height of the side sill 4 situated at the side of the rear end of the seat cushion 8a of a seat 8 to be installed in the passenger compartment 1b is made higher than the seat cushion 8a. By this constitution, when an occupant sits in the seat 8, the rear part of the side sill 4 which is higher than the seat cushion 8a of the seat 8 is situated at the side of the hip of the occupant.