Abstract: A hardening composition comprising a hardening component and hardening agents, said hardening component consisting essentially of magnesium oxide, said hardening agents consisting essentially of propylene carbonate and water, content of said magnesium oxide being 100 parts by weight, content of said propylene carbonate being 1 to 60 parts by weight, and content of said water being 20 to 135 parts by weight. Further disclosed are a hardened product prepared by hardening the hardening composition, and a method for producing the hardened product.