A leak check apparatus performs a leak check of a fuel vapor processing
apparatus based on a change in a negative pressure in a purge line during
leak down. The pressure change during leak down is corrected by measuring
an amount of vapor generated after the purge line is opened to the
atmosphere. A first pressure change rate of the purge line is measured
after a predetermined negative pressure is applied to the purge line, and
the purge line is sealed at the negative pressure. A second pressure
change rate of the purge line is measured after the first pressure change
rate of the purge line is measured, the cut valve is opened to expose the
purge line to the atmosphere, and the purge valve and the cut valve are
closed to seal the purge line at the atmospheric pressure. The second
pressure change rate measurement does not begin until the degree of
pressure increase in the purge line exceeds a predetermined reference
value.