To provide a hydraulic system, particularly, an oil pressure changer for the hydraulic system.

The oil pressure changer for controlling a hydraulic oil pressure and a flow rate for a hydraulic device comprises first and second variable flow gear pumps into which a high pressure hydraulic oil and a low pressure hydraulic oil are supplied for controlling a discharge pressure and a flow rate, if required. First gears for the first and second gear pump are located on a first shaft in common and second gears for the first and second gear pumps are located on a second shaft in common. The motion of the first shaft relative to the second shaft in the axial direction causes an increase or decrease in engagement length of the first and second gears for both the first and second gear pumps.