An initial liquid containing micro-bubbles is generated by mixing water and air (step S11). The bubble diameter for the micro-bubbles is 1 [mu]m or less. The bubble density of the initial liquid is measured (step S13), and if the density is less than a target density (step S14), heating and pressure reduction of the initial liquid are carried out and liquid is vaporized (step S15). With a reduction in the volume of the liquid, the density of the micro-bubbles increases, and a high density micro-bubble liquid is easily acquired. Alternatively, the high density micro-bubble liquid is easily acquired by increasing the density of the micro-bubbles in the initial liquid using a filter through which the micro-bubbles do not pass (step S15). If bubble density of the initial liquid is greater than the target density, the initial liquid is diluted (step S16).