The present invention discloses a supercooling promoting agent comprising a tannin for producing practical water which does not freeze. As the tannin, a hydrolyzable tannin such as 2,3,6-tri-O-galloyl-α,β-D-hamamelose, 1,2,6-tri-O-galloyl-β-D-glucose, and a vitrification liquid, each of which contains the supercooling promoting agent are useful as a solution or the like for storing a biological material at low temperature.
