PURPOSE: To increase a diagnozing chance without worsening reliability by accurately judging a steady operation condition of an engine during diagnosing a trouble.

CONSTITUTION: A change amount DALP with a value ALPSTT serving as the reference, when started a trouble diagnosis for a value ALP of the second parameter representing an operating condition, is obtained (S6), and when an absolute value DALP of this change amount is displaced from a preset width ALPSTD, the trouble diagnosis is suspended (S7, S12). During the trouble diagnosis, when the absolute value of this change amount is within the preset width, after elapsed the time, a mean value TPAVE of the first parameter value TP during diagnosis is judged (S10) for whether in a limit width or not between upper/lower limit values TPAVEH, TPAVEL, and when in the limit width, the diagnosis is discriminated to be performed under steady operation. Since the change amount of the second parameter value during the diagnosis is obtained by a relative fluctuation amount with a value serving as the reference when started the diagnosis, an influence of accuracy error or the like of sensors is prevented from being received even by small setting the preset width. Whether in the limit width or not is judged on the basis of the mean value during diagnosing the first parameter, and the limit width can be narrowed, to obtain increasing a diagnostic chance.