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A micro-bubble generator based on a pressurized dissolution method can generate fine micro-bubbles with a high bubble number density. The mean diameter and the number density of micro-bubbles depend on the concentration of dissolved gas in the upstream region of a decompression nozzle. In this study, the mean diameter and the number density of micro-bubbles in the downstream region of the decompression nozzle are measured by using a phase Doppler anemometry (PDA) to examine effects of the dissolved gas concentration on the mean diameter and the number density of micro-bubbles generated by the pressurized dissolution method. Experiments are conducted for several dissolved gas concentrations in the upstream region of the decompression nozzle under bubble cavitation and sheet cavitation conditions in the decompression nozzle. The experimental results indicate that, (1) influence of the dissolved gas concentration on flow patterns in the decompression nozzle (bubble cavitation and sheet cavitation) is weak, (2) the Sauter mean diameter of micro-bubbles increases as dissolved gas concentration increases, (3) the gradient of Sauter mean diameter with respect to the dissolved gas concentration in a sheet cavitation case is higher than that in a bubble cavitation case, (4) micro-bubbles are generated even when the dissolved gas concentration is lower than the saturated concentration at the atmospheric pressure provided that a sheet cavitation takes place at the nozzle, and (5) the bubble number density increases with the dissolved gas concentration in bubble cavitation cases whereas it does not depend on the concentration in sheet cavitation cases.