To provide a fuel tank of a working vehicle in which the fuel tank capable of increasing a fuel storage capacity without expanding outside a working vehicle body is installed on the side of a cabin installed via a mount section.

In a working vehicle, a cabin 8 is installed on a traveling vehicle body 3 via a mount section 9, and a fuel tank 2 is arranged at the side of the cabin 8. The fuel tank 2 comprises a main tank 2a of a rectangular shape in a plan view and a capacity-increased tank 2S close to the side wall 8a of the cabin 8 keeping clear of the mount section 9.