In a safety device of a fuel system for an internal combustion engine including a fuel filter with a venting nozzle adapted to prevent the discharge of fuel through the venting nozzle of the fuel filter which is connected to a pump line, the safety device which is mounted on the vent nozzle of the fuel filter includes in its housing a semi-permeable foil which is gas permeable but not liquid permeable. The semi-permeable foil extends across the vent nozzle passage and a non-return valve is disposed in the vent nozzle passage for closing the vent nozzle passage if the semi-permeable foil ruptures.