Abstract: A hydraulic apparatus realizes the same function as that of a variable discharge pump by regulating a hydraulic device such as a control valve in a state always operated at a substantially constant number of revolutions with a high efficiency regardless of the type of a hydraulic pump driven by a driving source such as a heat engine or electric motor. This hydraulic apparatus drives a hydraulic pump with a driving source internally or additionally provided with a predetermined amount of inertia, so as to construct a fixed pressure hydraulic source, and further provides peripheral devices corresponding to a required load, so as to open/close a control valve according to a state of a load including an energy accumulating device, a hydraulic motor, or the like such that the load can be supplied with an operating fluid ranging from a low flow rate at a high pressure to a high flow rate at a low pressure.