Abstract: A rotational speed parameter is detected in accordance with a rotational speed of the engine. A reference value of the rotational speed parameter is calculated. For each cylinder, an error between the reference value and the rotational speed parameter detected every predetermined crank angle is calculated as a relative speed parameter. For each cylinder, an integrated value is calculated by integrating the relative speed parameters over a predetermined period. For each cylinder, an average value per cylinder of the sum of the integrated values of all the cylinders is calculated. For each cylinder, an error between the integrated value of the cylinder and the average value is calculated. For each cylinder, the output of the engine is controlled in accordance with the error calculated for the cylinder such that variations in the output between the cylinders are suppressed.