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 an outlet pressure. In the hydraulic system, because the cavitation occurs often and causes an unpleasant noise as well as an instability in pressure-flow control, the cavitation avoidance is important. Hence, for cavitation avoidance under a high pressure condition, the influences of the specifications of the disk and nozzle as well as operating conditions on the cavitation and the inlet pressure change against the valve opening are experimentally investigated. Under a constant flow rate and an outlet pressure, the inlet pressure rises and the cavitation appears when the valve opening decreases. By decreasing the valve opening more, however, the cavitation disappears despite a rise in the inlet pressure. In addition, the round edge at the nozzle outlet is effective in avoiding the cavitation. These facts are demonstrated through measuring the pressure distribution in the disk valve. Finally, the cavitation boundary meaning the relationship between the outlet pressure and the critical inlet pressure at the cavitation beginning is investigated for various specifications of the disk valve and operating conditions.