PROBLEM TO BE SOLVED: To prevent the cavitation of a propeller by controlling switching by a signal proportional to engine speed, and an engine load signal. SOLUTION: An electronic control part 36 receives an RPM signal in a line 38 proportional to the rotating speed of a crankshaft of an engine 12 and also receives an engine load signal in a line 40 proportional to the load of the engine 12. The electric control part 36 generates a control signal to switch a transmission 16 to a high speed gear when both engine speed and engine load are relatively high, generates a control signal to a line 34 so as to switch the transmission 16 to a constant speed gear when both engine speed and engine load are relatively low, and generates a control signal to the line 34 so as to switch the transmission 16 to a low speed gear when the engine speed is medium and only engine load is high. The cavitation of a propeller in the constant speed gear can thereby be prevented.