According to one embodiment of the invention, a purge quantity is controlled in proportion to an air quantity sucked into an internal combustion engine. During normal operation of the engine, a purge air-fuel ratio is calculated according to a purge rate and an air-fuel feedback control quantity. During transient operation of the engine, a target air-fuel ratio feedback control quantity is calculated according to the purge rate and the purge air-fuel ratio. When the difference between the air-fuel ratio feedback control quantity and the target air-fuel ratio feedback control quantity is greater than a predetermined value, the air-fuel ratio feedback control quantity is corrected to the target air-fuel ratio feedback control quantity. An undue fluctuation in air-fuel ratio of a fuel mixture supplied to cylinders occurring upon rapid fluctuation in purge rate can be suppressed.