PURPOSE: To prevent the occurrence of stress concentration due to the sudden change of a section modulus by connecting reinforcing members to the inner side of a pair of body side frames having a U-shaped cross section so that part of the cross section opening part of the side frames can be remained to connect brackets supporting an engine to these reinforcing members.

CONSTITUTION: Side frames 2 have a U-shaped cross section, and reinforcing members 4 are fitted respectively to the inner sides of portions to which engine brackets 6 and 8 are connected. These portions have a half-opening cross section, and a section modulus is higher than that of a portion having a U-shaped cross section and is lower than in a case where a reinforcing part has a box state. Consequently a section modulus is not suddenly changed between a reinforced portion and an unreinforced portion to cause no stress concentration in a boundary line. Presser members 18 are connected to the tilted surfaces of the reinforcing members 4, and wiring and piping, etc., 22 arranged in the cross section opening part of the side frame 2 are pushed in the inside to be fixed. This work is to only fixing by the presser member 18, improving the efficiency of arranging work.