To perform a constant speed navigation at various speeds by controlling engine output so as to be gradually reduced according to the increase in engine rotating speed when the hull resistance is gradually reduced according to the increase in engine rotating speed when the engine rotating speed is constantly kept.

In the part where the hull resistance in A1 of hump state is gradually reduced, fuel injection quantity and ignition timing of ignition plug are set so as to be gradually reduced and delayed according to the increase in engine rotating speed, respectively. After the middle position of hump, the hull resistance starts to increase. Since the fuel injection quantity and the ignition timing of ignition plug are set there so as to be increased and advanced according to the increase in engine rotating speed, a ship can be prevented from being suddenly accelerated while keeping the throttle opening by reducing the engine output in the hump state. Thus, the feeling of physical disorder of an operator can be reduced.