PURPOSE: To simplify and thus realizing a compact structure by, in a fluid rotary actuator controlled by a spool valve, disposing a valve spool in an internal chamber of a rotation output shaft rotatably supported in a main body having a piston fitly mounted therein.

CONSTITUTION: In a servo actuator 10, a rotation output shaft 20 comprising as a whole a cylindrical central portion 22 having an internal chamber 24 is rotatably supported in a main body 12, and an external apparatus is coupled to an end flange 36 of the rotation output shaft 20. A cylindrical sleeve 40 is formed integrally with the end flange 36, and a piston sleeve 68 integrated with a piston 70 fitly mounted in an airtight chamber 62 in the main body is fitly mounted in a space between the sleeve 40 and the rotation output shaft 20, whereby a linear movement-to-rotary movement converting means is formed. Further, a valve spool 108 for controlling supply and exhaust of a pressurized fluid to and from each of chambers 80, 84 defined by the piston 70 is arranged in the internal chamber 24 of the output shaft 20.