To provide a fuel tank of a construction machine which can be mounted even in a narrow space and capable of ensuring effective fuel even when the amount of remaining fuel becomes small and the fuel tank is inclined.

Weirs 12 and 13 with their fore end sides being transversely bent in a hook shape are installed before and behind a suction port 11 formed in a center part of a tank body 10 in a flat shape to longitudinally partition the suction port 11. When the amount of fuel in a tank becomes small, and the tank is longitudinally inclined, the fuel of a small amount on the rear side or the front side flows into a fuel sump part 20 surrounded by the weirs 12, 13, and stored. A part of the stored fuel is ensured as effective fuel, the supply of the fuel is not tapped out even when the tank is inclined.