To easily produce an iodonium salt from an easily available raw material in high yield while avoiding strict requirement and reaction condition of the reaction material by contacting a mixture of an iodoaryl dicarboxylate and an acid.

A mixture of an iodoaryl dicarboxylate with molar equivalent of trifluoromethane sulfonic acid is brought into contact in a non-aromatic solvent at 0-100°C. The obtained homogeneous solution is brought into contact with a compound containing at least one aromatic nucleus having at most 5 substituents (having at least one unsubstituted H bonded to the aromatic nucleus) at 0-100°C for ≥30 min. The pure product can be separated by removing the reaction solvent, adding a non-solvent until the coagulation of the residue takes place and pulverizing the coagulate. The diaryliodonium salt is effective as a photoacid catalyst to be used in acid-sensitive polymerization and in a curable system such as a radiation-curable release coating composition.