PURPOSE: To obtain a great steering aiding force without the need of increasing the dimensions of a power steering unit by forming a female-screw engaging part in such a manner that the axis thereof is displaced from the center axis of the external diameter of a piston in the direction opposite to a sector gear.

CONSTITUTION: A piston 12 has the female-screw engaging part 16 formed on the internal peripheral face thereof in the position to have the axis of said part displaced by the specified distance S from the center of the external diameter in the reverse direction (that is, in the upper position in the drawing) with respect to the position of a sector gear 20. Therefore, without the needs of displacing the turning center O of the sector gear 20 into the lower position in the drawing and reducing the thickness of the piston 12, the radius R of the pitch circle of the sector gear 20 can be set to a value greater than that by the specified dimension and the steering aiding force can also be increased.