To provide a reinforcing structure of a vehicular cross section capable of improving installation work efficiency by arranging reinforcements outside a cabin, securing a larger space in the cabin by creating a ring skeleton structure reaching the outside of the cabin on the vehicular cross section in the arrangement positions of the reinforcements, and suppressing match box deformation at low cost.

A pair of right/left reinforcements 7L, 7R are adhered to each of outer wall surfaces of roof inside inner portions 4L, 4R of a vehicle 2 and upper-outer surfaces of wheel houses 3L, 3R. A cross-sectional structure in arrangement positions of the reinforcements 7L, 7R of the vehicle 2 forms the ring reinforcing structure by the right/left reinforcements 7L, 7R, a horizontal frame package tray 8 arranged between the right/left reinforcements 7L, 7R, the right/left wheel houses 3L, 3R positioned below the right/left reinforcements 7L, 7R, and a chassis 6 arranged between the right/left wheel houses 3L, 3R.