To provide a fuel tank which can limit an oil supply amount to a design value even when a tank body expands.SOLUTION: A fuel tank is provided with a breather pipe 5 which discharges air from an interior of a tank body 3 when oil is supplied. The breather pipe 5 penetrates through a tank upper surface 6 of the tank body 3 to the interior and the exterior. The breather pipe 5 is fixed to the tank upper surface 6. A position of a lower end opening 7 of the breather pipe 5 is set to an upper limit height 9 of a fuel 8. An expanding/contracting part 10 is provided at a portion, which is located within the tank body 3, of the breather pipe 5. A lower end part 12 of the breather pipe 5 is fixed to a tank bottom surface 13 of the tank body 3 through a bracket 11.SELECTED DRAWING: Figure 1