Provided is a fluoroboron compound which is highly safe and stable and is capable of forming a cyclic ether-fused ring by the intramolecular alkoxymethylation reaction, or a salt thereof. The compound can be synthesized by the intramolecular alkoxymethylation reaction of a fluoroboron compound represented by the formula (I) or a salt thereof in the presence of a metal catalyst. (wherein the moiety represented by the formula represents an aromatic ring; L represents a substituent such as a halogen atom; R represents a substituted or unsubstituted alkylene group having 1 or 2 carbon atoms; and M represents an alkali metal cation or the like, with the proviso that L and —R—OCH2BF3M are respectively located on contiguous carbon atoms on the aromatic ring, or in the case of a fused aromatic ring, on two carbon atoms adjacent to one carbon at the fused position).