A spool valve for water hydraulics, Aqua Drive System (ADS), which uses tap water, has some rectangular openings at control ports for precise control because of the low viscosity of the water. In the valve, cavitation happens quickly, and it occurs some troubles, e. g. noise and vibration, erosion, degradation of controllability. This study focuses on cavitation behavior and cavitation-related noise and pressure-flow rate characteristics of a rectangular orifice. Jet flow behavior issuing from a rectangular orifice strongly depends on its aspect ratio. So, cavitation could be affected by the aspect ratio. However, there is little knowledge of the relationships between the aspect ratio and cavitation noise or pressure-flow rate. This research investigates the aspect ratio influence on cavitation, noise, and pressure-flow rate characteristics with different pressure conditions. The equivalent diameter of the orifice used in this study is from 0.35 to 1.3mm, and it is based on a product spool valve model. As a result, the cavitation jet behavior changes with the orifice aspect ratio and the ambient pressure. But the noise characteristics show the almost same tendency for different aspect ratio.