To provide a new method for synthesizing a macrocyclic compound through reaction between 1,3-dioxolane and ethylene oxide.

This method for synthesizing a cyclic ether formal with its ring size enlarged by oxyethylene unit compared to that of 1,3-dioxolane as a reactant comprises reaction between 1,3-dioxolane and ethylene oxide in the presence of a cationic catalyst which is pref. a cationic polymerization catalyst used in ring opening polymerization such as boron trifluoride or an ether complex thereof, or e.g. a well-known cationic catalyst such as a heteropolyacid, trifluoromethanesulfonic acid or trifluoromethanesulfonic anhydride.