This article describes cavitation in a hydraulic disk valve to control an inlet pressure by changing a valve opening at a constant flow rate and outlet pressure. A disk valve, which has a fundamental configuration of a poppet valve, is widely used in a hydraulic system such a power steering system in order to control flow and pressure in the system. The disk valve, which makes a large flow area with a small valve opening, has a simple construction and does not reqire its parts to be machined with severe torelances. The disk valve with a diverging flow, however, is apt to cause cavitation because the inlet pressure becomes lower than the outlet pressure in a gap between the nozzle seat and the disk. The cavitation affects the pressure control performance and thrust force and causes an unpleasant noise. Hence, the cavitation is a very important problem because the hydraulic system is used recently under a high pressure condition. Therefore, the effects of the specifications of the disk valve such as a nozzle inner diameter, roundness of the nozzle outlet and an outer diameter of the disk, and operating conditions such as the flow rate and the outlet pressure in the disk valve on the cavitation and the inlet pressure change against the valve opening were experimentally investigated. When the valve opening was decreased under a constant flow rate, the inlet pressure rose and the cavitation which appeared initially near the nozzle outlet spread out in the entire gap. By decreasing the valve opening more, however, the cavitation disappeared despite a rise in the inlet pressure. In addition, a round edge at the nozzle outlet was evidently effective in restriction of the cavitation occurrence because it prevents from making the flow the contraction. In this work, these facts were demonstrated through results of the measurement of the pressure distribution in the gap as well as the visualization test. Including this fact, the cavitation boundary meaning the relationship between the outlet pressure and the inlet pressure at the cavitation beginning was investigated. The results obtained from this work would be successfully applied to the poppet valve with a large annular area bacause both the valves have a similar flow pattern.