Effects of liquid volumetric flux J_L and flow pattern in a decompression nozzle on the mean diameter and number density of micro-bubbles generated by using a pressurized dissolution method are experimentally investigated. The bubble diameter and number density are measured by using a phase Doppler anemometry at the exit of the decompression nozzle. The flow pattern in the decompression nozzle was observed by using a high speed camera. The experimental results indicate that the diameter and number density strongly depends on the flow pattern in the decompression nozzle, and their characteristics are classified into three regimes: (1) few bubbles are generated when no cavitation occurs, (2) when bubble cavitation takes place, the mean bubble diameter keeps low values which do not depend on J_L so much and the bubble number density increases with J_L, and (3) the mean bubble diameter increases with J_L whereas the number density does not depend on J_L when flow pattern in the nozzle is sheet cavitation.