During the investigation of the mechanism that cellular water in woody plants growing in cold districts keeps liquid state at low temperature, the inventors have studied to identify the causative substances. As the results, the present inventors identified supercooling promoting agents in woody plants. The supercooling ability of identified flavonoid glycoside and synthesized flavonoid glycoside with similar structure was tested. It was found that the supercooling promoting agent comprising these flavonoid glycosides enables to stably supercool bulk water at low temperature for long-term. The aqueous solution containing the supercooling promoting agent of the present invention is useful to store biological materials at low temperature.