PURPOSE: To make improvements in controllability by detecting a fact that a throttle is operated to its opening side, thereby altering the pitch angle of a propeller to the specified pitch angle as well as detecting a fact that it is operated to the closing side, thereby altering the propeller pitch angle to the minus side rather than the specified pitch angle.

CONSTITUTION: A setting signal is outputted from a computing element 83 inputting each detecting signal of an engine speed detector 81 and a throttle opening detector 82, and this setting signal is compared with a measuring signal of an actual pitch angle measured by a potentiometer 85 at a controller 84. A servomotor 86 is controlled by a control signal based on this compared result, whereby the pitch angle of a propeller is altered. In brief, in the case where a throttle is opened more than the setting opening and engine speed has exceeded the upper limit of a speed setting value, the pitch angle is altered to the plus direction. On the other hand, in the case where the throttle is closed from the setting opening and the engine speed goes higher than the lower limit of the speed setting value, the pitch angle is altered toward the minus direction.