To provide a fuel tank of a construction machine capable of easily maintaining fuel in the fuel tank in a clean state.

The fuel tank 34 comprises a tank body 40 having a fitting hole 60 in an upper wall 58, and a fuel filling port assembly 62 which is attachably/detachably fitted to the upper wall 58 of the tank body 40 with the fitting hole 60 being closed, and includes a filter unit 70 which is arranged in the tank body 40 through the fitting hole 60, and introduces the fuel into the tank body 40 by passing the fuel filled in from a fuel filler tube 36. The filter unit 70 includes a filter case 100 having a fuel introduction port 106 into the tank body 40, and a bag-shaped filter 114 which is attachably/detachably stored in the filter case 100 to filter the fuel. The filter 114 is formed of a sheet-like filter material.