To provide a rear wheel house reinforcing structure which can suppress increase in the number of component parts, give the desired torsional rigidity, and is free of heightening of the center of gravity.

A part 24 having a closed section extending across the car width is formed at one corner 17a of the lower part of a rear seat back center panel 17 and on the oversurface 2a of a floor panel 2. Through a left 28 and a right rear suspension mount bracket 28, the left end 17i and right end 17i of the closed section part 24 are coupled with suspension bracket upper's 3b of a left 3 and a right rear wheel house 3.