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Abstract: In recent years, many ultrafine bubble (UFB) water production methods have been developed. Recently, a batch-type UFB water production method by rapidly cooling and condensing a steam and non-condensable gas mixture has been invented. In this study, we improved the method above and developed a continuous-type UFB water production method. UFB water was produced by continuously cooling and condensing the steam and non-condensable gas mixture with a stainless-steel coaxial double-pipe condenser. By changing parameters such as cooling water flow rate, gas flow rate, and steam flow rate, we investigated the effect of parameter changes on the number concentration of UFB water produced. We found that the number concentration increased as the cooling water flow rate increased, decreased as the gas flow rate increased, and remained almost constant as the steam flow rate increased. These results were discussed from the perspective of heat transfer, and it was suggested that a correlation could be established between the amount of heat transferred and the number concentration.