A fuel supply system for supplying a fuel to an engine mounted on an upper frame of heavy construction/forest equipment is provided. The fuel supply system includes a junction tank unit for interflowing a fuel supply line which is connected to a lower end of a primary fuel tank and a fuel supply line which is connected to a lower end of a secondary fuel tank, the fuel supply lines being connected in parallel to each other, a water separator connected to the junction tank unit via a fuel supply line, a fuel supply line for supplying the fuel to an engine from the water separator, and a fuel recovery line for recovering a remaining fuel from the engine to the primary fuel tank.