A method of selectively producing the syn-isomer of a compound having a hydantoin ring, which is important as an optical material, an electronic material, an active ingredient and an intermediate of medicines and pesticides and an active ingredient and an intermediate of diagnostics, comprising converting a compound having a hydantoin ring (for example, (E,Z)-2-benzyloxy-5,7-diazaspiro[3,4]octan-6,8-dione represented by the following formula (1)) into a t-butyl ester derivative thereof (formula (2)), crystallizing the same to selectively give the syn-monoBoc isomer thereof and then leaving the t-butyloxycarbonyl group.