An object of the present invention is to provide a process for synthesizing a vitamin D compound by simple procedures at lower costs.The present invention provides a process for preparing a vitamin D compound and an intermediate thereof, comprising the step of: (a) mixing a ketone or aldehyde, a Wittig reagent, and a base or (b) mixing a ketone or aldehyde and a Wittig reagent, and then adding a base to the resulting mixture.