To provide a compact and highly reliable electrohydraulic servo motor having reduced shaft length despite the adoption of a spool valve.

This electrohydraulic servo motor is provided with a first helical gear 44 connected with an output shaft 34, a second helical gear 45 engaged with the first helical gear 44, a spool 54, which is rotatably connected with the second helical gear 45 and is moved in the axial direction by the second helical gear 45 in order to regulate the communicating state between a working oil discharge passage 31a or a working oil supply passage 31b with a circular arc hole 40a of a valve plate 40, and a motor 50 having a revolving shaft 51 threadedly coupled with the second helical gear 45.