A novel technique is provided for preserving a liquid pharmaceutical preparation safely and stably. The nano-bubble water of the present invention, which is produced in the presence of a surfactant, a hydrophilic resin and/or an electrolyte and contains not less than 2.0×108 bubbles/mL of nano-bubbles, shows a superior antibacterial action, and can be used as a base of a liquid pharmaceutical preparation showing safe and stable preservation property.