To improve the rigidity of a body lower part.

Each lateral end of a floor panel 1 constituting a cabin floor surface is connected to a side sill 2, and a longitudinally extending tunnel part 3 is formed in the lateral center thereof. A lateral pair of floor frames 4 is connected to the lower surface of the floor panel 1 between the side sill 2 and the tunnel part 3. A first cross member 11 is arranged in the front between the floor frames 4 on the lower surface of the floor panel 1, and a second cross member 12 is arranged in the rear. Both the cross members 11 and 12 are mutually connected through laterally divided front reinforcing members 21 and 22. The second cross member 12 is connected to the floor frame 4 in the rear thereof through laterally divided rear reinforcing members 31 and 32. Each of the members 11, 12, 21, 22, 31 and 32 can be constituted as a preliminarily integrated set body and mounted on the body through fixtures in connection parts R1-R3 by post installation.