PURPOSE: To prevent wear of an adjusting shaft by placing a torsion bar spring for producing return force inside the adjusting shaft and connecting one end of the spring to an adjuster mechanism casing and the other to the shaft.

CONSTITUTION: When an adjusting shaft 30 rotates on an end 47b of a torsion bar spring 47 projecting into an adjuster mechanism chamber 36, the spring 47 is twisted by the shaft 30 to exert return moment linearly proportional to the angle of torsion of itself on the shaft 30. The shaft 30 is pressed by the pressure within a pump internal chamber 9 into the chamber 36, where it is supported on an adjuster mechanism casing 37 via the spring 47 and an adjusting bolt 50. The spring 47 is fastened immovably via the end 47b projecting into the chamber 36 against any fastened-part movement during turning motion of the shaft 30, so as not to cause its contact pressure to the shaft 30 to produce the wear moment that checks the turning motion of the shaft 30.