The present invention relates to a method of protecting a hydroxyl group, which includes reacting a hydroxyl group-containing compound with a compound represented by the formula (I): wherein R is a phenyl group optionally having substituent(s), an alkyl group optionally having substituent(s) or a benzyl group optionally having substituent(s), and X is a halogen atom, in the presence of an acid catalyst to substitute the hydrogen atom of the hydroxyl group of the hydroxyl group-containing compound with a protecting group represented by the formula (II): wherein R is as defined above. The present invention provides a method capable of introducing an acetal type protecting group into a hydroxyl group under mild conditions, and a protecting reagent therefor and a method of producing the protecting reagent.