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-±, ²-D-hamamelose, 1, 2, 6-tri-O-galloyl-²-D-glucose, and a vitrification liquid, each of which contains the supercooling promoting agent are useful as a solution or the like for storig a biological material at low temperature,