To obtain the subject compound useful as an intermediate for producing a therapeutic agent for dermatosis, by irradiating a specific provitamin D3 derivative with ultraviolet rays, separating an unreacted substance and thermally isomerizing the resultant substance.

A provitamin D3 derivative of formula I (Z and Z' are each H or an OH-protecting group with the proviso that a case in which Z and Z' are both H is omitted) is irradiated with ultraviolet light to give a mixture of a provitamin D3 derivative of formula II and an unreacted provitamin derivative of formula I. The unreacted provitamin D3 derivative of formula I is separated from the mixture by crystallization using an organic solvent and provitamin D3 derivative of formula II is thermally isomerized to give a provitamin D3 derivative of formula III. The separated unreacted provitamin D3 derivative of formula I is circulated and reused.