Image Number 3 for United States Patent #7036354.
A pump module 30 is provided for introducing a negative pressure t a fuel tank 10. A tank's internal pressure at a time, upon which a steady-state pressure reaching time elapses after introduction of the negative pressure is started, is detected as a convergence value. It is determined whether or not there is a leak in a system including the fuel tank based on that convergence value. A space volume inside the fuel tank 10 is detected by an output produced by a fuel level gauge 12. The steady-state pressure reaching time is set based on the space volume.