To avoid generation of unstable vibration caused by resonance phenomenon.

A hydraulic servo mechanism is equipped with a hydraulic servo system S, an oil supplying device 1 for supplying drive oil to the servo system S and pipelines P1 and P2, and supplies drive oil from the oil supplying device 1 to the servo system S via a hydraulic damper device (accumulator) 2, wherein an oil line length changing means (resonance avoiding circuit) 4 is furnished to change the oil line length between the servo system S and damper device 2, and unstable vibration likely generated in the servo mechanism is avoided by changing the oil line length using the means 4.