This invention provides a solution for preserving mammalian early embryos or ES cells by vitrification, which comprises, as a base material, a phosphate buffer that exclusively contains 10% to 15% (v/v) propylene glycol as polyhydric alcohol or a phosphate buffer that exclusively contains 10% to 15% (v/v) propylene glycol and 25% to 35% (v/v) ethylene glycol as polyhydric alcohols and further contains 15% to 25% (v/v) Percoll® and 0.2 M to 0.5 M sucrose. This invention also provides a method for preserving mammalian early embryos or ES cells by vitrification using such solution.