An acetalsulfonate derivative useful as an intermediate for medicines and agricultural chemicals; a process for industrially producing the derivative; and a process for industrially producing a styrene oxide derivative from the acetalsulfonate derivative or from a mandelic acid derivative. The process for producing an acetalsulfonate derivative comprises: a first step in which a mandelic acid derivative is esterified to form a mandelic ester derivative; a second step in which the mandelic ester derivative is protected by an acetal to form an acetal derivative; a third step in which the acetal derivative is reduced to form an ethanediol derivative; and a fourth step in which the ethanediol derivative is reacted with a sulfonyl chloride derivative. The process for producing a styrene oxide derivative comprises the step of deacetalizing the acetalsulfonate derivative and the step of epoxidizing the thus-treated sulfonate derivative with the aid of a base catalyst.