This invention performs a diagnosis on the evaporative system in a short period of time by pulling down the evaporative system with a negative pressure of the intake manifold of the engine and by checking a change in the internal pressure of the evaporative system being pulled down. To realize this diagnosis, when introducing a negative pressure from the intake manifold, the pulldown rate is changed based on the internal pressure of the evaporative system. Because the pulldown rate is corrected and controlled during the diagnostic process, a preliminary process for the diagnosis can be eliminated, shortening the time required for the diagnosis. Further, this method allows the evaporative system to be diagnosed without being influenced by external factors such as atmospheric pressure, the internal volume of the evaporative system, the amount of evaporative gas generated, or presence or absence of leakage.