To provide a servo motor for keeping a difference between the rotating speed of a fluid motor and a rotating speed instructed by an input signal smaller than conventional, even when torque generated by a fluid motor is smaller than loading inertia.

The electric hydraulic servo motor 100 comprises a rotation signal generating part 310 for generating a rotation determining signal 114 for determining the condition of rotating an electric motor 210 in accordance with the input signal 111 to be input, the rotating speed 106 of a hydraulic motor 220 and the pressure of an operating oil 101 to be supplied from a guide valve 244 to the hydraulic motor 220 and inputting it to the electric motor 210, a stop signal generating part 320, an electric motor signal input part 330, and a connection change-over part 340.