PURPOSE: To prolong the service life of a fluid actuator inside contaminated operating environments so as to seal against the ingress of contaminants of the high pressure seal area.

CONSTITUTION: To an actuator 10 consisting of a housing 12 having a cylindrical hollow hole 14, a piston 16 slidably arranged inside the hollow hole 14, an end wall 18, and a sealing plug 20, a seal assembly 24 for preventing the leakage of a pressurized fluid through the clearance around a shaft 22 from the inside of the housing 12 is provided, and the seal assembly 24 in which primary and secondary scrapers 26, 28 are arranged in the radial inside of the sealing plug 20 is constituted by a terminal packing, a static seal and a dynamic seal, so that the pressurized operating fluid is prevented from leaking out to the outside of the housing 12 through an volume 36 after wards.