Rattling noise of the final gear is one of the important items of noise quality which is attributed to the torsional vibration of the drive line. This noise is originally generated by the impacts of final gear pair which has an indispensable backlash. Accordingly, the excitation force of this noise is the rotational fluctuation of the drive pinion. Prediction of the rotational fluctuation of the drive pinion at the design stage is one important factor for analyzing the expected level of rattle. In spite of many attempts at prediction, the accuracy of those methods has remained insufficient. In this paper, a new method using digital simulation is introduced. The accuracy was improved by taking the backlash between spline sleeve of the output shaft of the gear box and the propellor shaft into consideration. This new method enabled prediction of the rotational fluctuation of the drive pinion with sufficient accuracy. For the covering abstract see IRRD 870290.