A cetane number estimation method is provided in which: the preliminary injection is preformed multiple times at different compression end temperatures with a fuel tank (26) containing fuel having a predetermined cetane number, and an engine torque increase caused by each preliminary injection is calculated and the relation between the compression end temperatures at the respective preliminary injections and the engine torque increases caused by the respective preliminary injections is determined; the compression end temperature and the engine torque increase at a predetermined reference point on the relation are recorded as a basic compression eng temperature and a basic engine toque increase; the preliminary injection is performed at the basic compression end temperature and an engine torque increase caused by this preliminary injection is calculated; and the cetane number of fuel is estimated based on the relation between the calculated engine torque increase and the basic engine torque increase.