To ensure high support rigidity in a support structure of a power unit and improve the control of dropping locus of a rear part of the power unit in a frontal collision.

An X type cross member 4 having substantially X-shaped plane shape is used as a cross member supporting the rear part of the power unit 1. The X type cross member 4 is attached to side frames 2 on the left and right sides in four sections including left and right front side mounting points b, b and left and right rear side mounting points c, c for a power unit support point a of the X type cross member 4 by mounting bolts. Dropping load of the left and right front side mounting points b, b is set to be lower than dropping load of the left and right rear side mounting points c, c. The left and right front side mounting points b, b are previously broken and drop when front collision load is inputted. The X type cross member 4 turns downward centered on a line c-c connecting the left and right rear side mounting points or its vicinity. The rear part of the power unit 1 is securely displaced downward along controlled turn locus of the X type cross member.