To provide servo actuator for improving energy efficiency.

This invented servo actuator is equipped with an electric motor section 20 rotating output shaft 22, a rotational transducer 30 converting rotation of the output shaft into axial displacement of main shaft 1, and a case 40 to which above these components are attached. The rotational transducer 30 has a ball screw mechanism 50 and a guidance mechanism 55, which inhibits the rotation of the ball screw mechanism 50, while allowing transfer towards axial direction. In the guidance mechanism 55, rails 34 having a pair of rail faces 34a formed on two planes which substantially orthogonally-cross each other at the center of the main shaft and a pair of guide units 33, comprising a pair of rollers 33a attached to ball screw nut 31 so as to roll on each rail face, are arranged symmetrically by sandwiching the main shaft.