PURPOSE: To relieve a boss of external adjustment force by forming an adjustment ring having an adjustment rod for propeller blades in a ring cylinder and axially arranging it in a cylinder chamber of the boss so that it can be acted upon hydraulically on both sides.

CONSTITUTION: The adjusting device for an adjusting rod 16 is composed of a ring piston 17 fixed to the left end of the adjusting rod 16 for making adjustments of a propeller blade 13 and a cylinder chamber 18 axially extending toward a boss 4. The ring piston 17 is then arranged in the cylinder chamber 18 actable on both sides. Hydraulic fluid is supplied to acting chambers 22, 23 of the cylinder chamber 18 through a hole 24 on an outer boss housing 6 and the propeller blade 13 is adjusted by sliding the ring piston 17 inside the cylinder chamber 18. This composition enables all adjusting forces to be actuated only in the interior of the ring cylinder chamber and the boss can be relieved of external adjustment force.