formed by two pipes 3 the upper pipe and the lower pipe are arranged so as to be at an approximately equal distance from the central position 33 in the height direction of the side frame 2 with the central position 33 being held between the two pipes as shown in fig1 and 2 the side frame mounting member 7 is formed so as to extend from an upper part of the pipe fixing member 6 toward the rear of the vehicle at the rear end of the side frame mounting member 7 an engagement portion 11 is provided this engagement portion 11 is formed by bending a rear end portion of the side frame mounting member 7 downward also the side frame mounting member 7 is provided with a hole 12 refer to fig2 on the front side of the engagement portion 11 on the other hand in a top surface 2 a of the side frame 2 shown in fig1 an engagement hole 13 is provided so that the engagement portion 11 of the side frame mounting member 7 engages with the engagement hole 13 also in the top surface 2 a a mounting screw hole 9 is provided on the front side of the engagement hole 13 this screw hole 9 is provided so as to align with the hole 12 in the side frame mounting member 7 in the state in which the engagement portion 11 is engaged with the engagement hole 13 also as shown in fig1 in the tip end portion on the front side of the side frame 2 an attachment member 10 is provided so as to cover the opening of the square steel pipe of the side frame 2 at a lower part of the attachment member 10 screw.