When a speed difference between a maximum rotation speed and an engine rotation speed, that is, an actual rotation speed difference, is equal to or less than a margin rotation speed difference, an engine operating point is changed such that the actual rotation speed difference becomes greater than the margin rotation speed difference. Accordingly, the speed difference between the maximum rotation speed and the engine rotation speed is prevented from becoming equal to or less than the margin rotation speed difference. As a result, since a relatively sufficient margin is secured in the difference between the maximum rotation speed and the engine rotation speed, it is possible to prevent the engine rotation speed from falling into a high-rotation state in which the engine rotation speed exceeds the maximum rotation speed.