To prevent heating of a fuel tank by atmosphere in a space in car body lower part structure of a vehicle forming the space projected upward by raising a floor panel, arranging the fuel tank in the space and opening a rear end of a tunnel part from a lower part of a raising part of the floor panel against the space.

This vehicle is constituted so that a floor panel 1 is raised and a space 3 projected upward is formed, a fuel tank 9 is arranged in the space and a rear end of a tunnel part 6 is opened from a lower part of a raised part 1a of the floor panel 1 against the space 3. The fuel tank 9 is arranged with a constant interval against the raised part 1a of the floor panel 1, and a rear end part of the tunnel part 6 and the fuel tank 9 are connected to each other through a plate member 16 so as to restrain inflow of hot air 20 into the space 3 from the tunnel part 6.