A hydraulic system (1A-1E) according to the present invention includes a rotational shaft (5), a hydraulic rotor (6) supported by the rotational shaft (5) so as to be incapable of relative rotation, an output adjuster (7) varying a volume of the hydraulic rotor (6), a control shaft (40) tilting the output adjuster (7), a drive motor (110) operating the control shaft (40) around the axis, a drive shaft(120) disposed on a reference axis (X) same as the axis of the control shaft (40) and operably driven around the axis by the drive motor (110), a deceleration mechanism (200) decelerating rotary power input from the drive shaft (110) and outputs the decelerated rotary power toward the control shaft (40), and a housing (50) accommodating the hydraulic rotor (6), the output adjuster (7), and the deceleration mechanism (200), wherein the deceleration mechanism (200) has a hypocycloid-type deceleration mechanism (300) disposed on the reference axis (X).