PURPOSE: To secure sufficient strength against a collision and reduce the weight.

CONSTITUTION: A left front side frame 2 is divided into a front frame section 2a and a rear frame section 2b press-machined with a thin plate respectively. A front member 6 having a nearly C-shaped cross section and a side wall member 7 blocking its opening face are spot-welded at the mating connection flange sections to form the front frame section 2a, a rear member 8 having a nearly U-shaped cross section and an upper wall member 9 blocking its opening face are likewise connected to form the rear frame section 2b, and they are formed into a rectangular cross sectional shape respectively. The press direction of the rear frame section 2b is made vertical, the connection flange section protruded from the lower face is eliminated, and bead sections can be formed on both the right and left side faces of the front frame section 2a. The buckling strength of the front end section of the front side frame can be increased without changing the thickness, the longitudinal strength can be controlled, and the weight can be reduced.