Disclosed herein is an .alpha.-methylene-cyclopentanone derivative represented by the formula [I] below which is useful as an intermediate for pharmaceuticals and insecticides, especially prostaglandins. Also disclosed herein is a process for producing the derivative advantageously on an industrial scale. ##STR1## where X denotes (.alpha.-OZ, .beta.-H) or (.alpha.-H, .beta.-OZ), with Z representing a protecting group for the hydroxyl group; U denotes (.alpha.-H, .beta.-R.sup.1) or (.beta.-R.sup.1, .alpha.-H); R.sup.1 denotes ##STR2## where R.sup.2 denotes a protected hydroxyl group, a substituted or unsubstituted C.sub.1-15 alkyl group, a substituted or unsubstituted C.sub.2-15 alkenyl group, a substituted or unsubstituted C.sub.2-15 alkynyl group, or a substituted or unsubstituted C.sub.6-15 aryl group; Z' denotes a protecting group for the hydroxyl group; and k is 0 or 1).