PURPOSE: To obtain a steroid derivative useful as a synthetic intermediate for hydroxyvitamin D derivatives in a short process in a high yield by reacting a specific carbaldehyde derivative with a methyl ketone in the presence of a basic substance.

CONSTITUTION: A carbaldehyde of formula of formula I (R3 and R4 are H or OH-protecting group) is reacted with a methyl ketone of formula II (R1 and R2 are lower alkyl or form a ring with a carbon atom bonded to R1 and R2) and optionally the OH group is protected or deprotected to give a hydroxyketone derivative of formula III (R5 and R6 are H or OH-protecting group), which is dehydrated in the presence of an acidic substance, a basic substance or a dehydrating agent to give a hydroxyketone derivative and optionally the OH group is protected or deprotected to give the derivative of formula IV (R7 and R8 are H or an OH group protecting group) [e.g. 26,27-dehydro-1α,3β- bis(methoxycarbonyloxy)cholest-5,7,22-trien-24-one].