A fault diagnosis apparatus for a fuel evaporative emission suppressing system has an electronic control unit which inputs an average value of integral terms for air-fuel ratio feedback control, engine speed, etc. when diagnosis executing conditions are satisfied, and then starts opening operation of a purge control valve. Subsequently, the average value of integral terms, engine speed, etc. are input again. If no substantial change occurs in the average value, etc. with driving of the purge control valve, it is concluded that purge air for fault diagnosis has not been introduced, and that the suppressing system is faulty. In driving the purge control valve, its driving duty ratio is increased by a relatively small increment till the driving duty ratio reaches a predetermined duty ratio. If the system is normal, therefore, a purge-air introduction amount for fault diagnosis is increased by a relatively small increasing degree, to thereby prevent fluctuation of the air-fuel ratio or engine output torque attributable to the purge air introduction. After the driving duty ratio has reached the predetermined duty ratio, the driving duty ratio is increased by a relatively large increment, to thereby rapidly execute the purge-air introduction and fault diagnosis.