Abstract: A device for detecting leakage in a fuel supply system of an internal combustion engine in which fuel is pumped from a fuel container under pressure by at least one pump and reaches injection valves over corresponding lines, the device has a control unit, a plurality of operating condition detecting sensors detecting operating conditions of the internal combustion engine and sending signals to the control unit, a return line with a valve formed so that excess fuel is returnable to the fuel container, the valve being connected with the control unit so that when the control unit detects a predetermined operating state of the engine from the signals of the operating condition sensors, the control unit closes the valve, and a pressure sensor, and a pressure sensor operative for measuring a fuel pressure when the valve is closed and connected to the control unit so that the pressure sensor supplies to the control unit a signal corresponding to the measured pressure, and the control unit compares the signal corresp