Fault diagnosis apparatus of fuel evaporation/dissipation prevention system

When a first restoring pressure amount, measured after a fault diagnosis object region is in a reduced pressure state, exceeds a first or second judgment value, a second restoring pressure amount is measured by releasing the fault diagnosis object region to the atmosphere and then sealing it. When the first restoring pressure amount is between the first and second judgment values, the second restoring pressure amount is compared with a third judgment value. When the first restoring pressure amount exceeds the second judgment value, the second restoring pressure amount is compared with a fourth judgment value. Leak is determined to exist when the first restoring pressure amount exceeds the first judgment value and the second restoring pressure amount does not exceed the third judgment value, or when the first restoring pressure amount exceeds the second judgment value and the second restoring pressure amount does not exceed the fourth judgment value.