PURPOSE: To provide an apparatus for diagnosing failures in an evaporation purge system, which cam restrain emission from degradation due to the introduction of negative pressure in a small duty ratio, while preventing the judgement of failures from error due to pressure pulsation.

CONSTITUTION: A negative pressure introduction controlling means 8 for opening and closing a purging controlling valve 7 by duty control, a pressure detecting means 9 for detecting negative pressure in a purge path 6 and a judging means 10 for judging whether or not a pressure value detected by the pressure detecting means 9 during a period of diagnosing failures continues exceeding a specific negative pressure arrival judging value within a specific time judging value to get smaller than a specific negative pressure judging value (large as negative pressure) are provided.