PURPOSE: To transmit load input in a tunnel part to a floor panel so as to improve buckling strength, etc., of the floor tunnel part by forming both shelf parts, etc., as load transmission means projecting to outward in the car lateral direction respectively in left/right both vertical walls of the floor tunnel part.

CONSTITUTION: A floor tunnel part 12 which is formed in the central part of a floor panel 10 in the car lateral direction extends along the central line (P) in the car body longitudinal direction and is formed upward into a projecting part. Both shelf parts 14, 16 as load transmission means, which projects to outward in the respective car lateral direction, are formed in left/right both vertical wall parts 12A, 12B of the floor tunnel part 12. After load input in the front end upper part of the floor tunnel part 12 is transmitted from respective upper parts 14A, 16A of both shelf parts 14, 16 to respective lower parts 14C, 16C, it is transmitted to the general face 10a of the floor panel 10. Namely, the load input in the front end upper part of the floor tunnel part 12 is received by tensile strength inside the general face 10A. Buckling strength of the floor tunnel part 12 and the car body strength can be improved.