The present invention is to provide a method for preparing easily and efficiently β-butyrolactones and/or optically active 3-hydroxycarboxylic acid derivatives in high optical purity by use of an easily available hydrolase, which are useful as intermediates for pharmaceuticals, agrochemicals and the like. More particularly, the present invention relates to a method for preparing β-butyrolactones and/or optically active 3-hydroxycarboxylic acid derivatives with an optical purity of substantially 100% ee, comprising reacting a β-butyrolactone which is a mixture of optical isomers, with a nucleophilic agent in the presence of a hydrolase, provided that a lipase derived from porcine pancreas is excluded.