To obtain an activated vitamin D derivative having an excellent bone quantity-improving action, an excellent differentiation inducing action, an excellent cell proliferation-inhibiting action, an excellent immunoregulatory action, etc., without causing hypercalcemia.

A compound of the formula (A and B are each carbonyl or methylene), for example, (24R,25R)-1α,3β,25-trihydroxyvitamin D4 26,28-lactone. The compound of the formula is obtained by selectively dehydrating the tertiary hydroxyl group of a phenylsulfone derivative, coupling the obtained compound with the 4-phenyl-1,2,4-triazoline-3,4-dione adduct of a 22-iodo steroid in the presence of a base, converting the reaction product into a side chain triol derivative, converting the compound into a 26-carboxylic acid derivative, hydrolyzing the derivative, irradiating the obtained precursor of the compound of the formula with UV light, and subsequently thermally isomerizing the product. The compound of the formula is preferably administered at a daily dose of 0.1-100 μg.