An agent for optical resolution comprising a bicyclo [3.3.0] -2-oxaoctane compound represented by the formula [1], [2] or [3] ; a process for producing an optically active substance comprising obtaining a mixture of diastereomers by reaction of the agent with a mixture of optically active substances having active hydrogen atom, resolving the mixture of diastereomers into each diastereomer and obtaining an (R) optically active substance or an (S) optically active substance by decomposition of the diastereomer; and compounds represented by the above formulae in which R 11 represents fluorenylmethyl group, fluorenylidenemethyl group, bis(4-cyclohexylphenyl)methyl group, 4-(9-phenanthryl)phenyl group, 4-(1-pyrenyl)phenyl group, 4-(5-acenaphthenyl)phenyl group or 4-(9-anthryl)phenyl group, are disclosed. A useful optically active substance having a very high purity can be produced efficiently by using the agent of the present invention.