Disclosed is a vinamidinium salt represented by the formula (I): [wherein R1 represents a hydrogen atom, a halogen atom, an optionally substituted hydrocarbon group or the like; R2 and R3 are the same or different and each represents a lower alkyl group, or alternatively R2 and R3 may combine together to form a ring; R4 and R5 are the same or different and each represents a lower alkyl group, or alternatively R4 and R5 may combine together to form a ring; X and Y are the same or different and each represents a bromine atom or a chlorine atom; n represents a number of not less than 2; and m represents 0 or n], and methods for synthesizing a heterocyclic ring compound using the salt.