PURPOSE: To produce the subject high-purity compound useful as an intermediate for optical materials, etc., in high yield by reacting 3,4,5,6- tetrafluorophthalonitruile with a specific nuclear substitution product with a basic substance in a specified solvent, then adding water thereto and separating an organic layer.

CONSTITUTION: (C) 3,4,5,6-Tetrafluorophthalonitrile is reacted with (D) a compound of the formula HX-R-YH (X and Y are each O or S; R is 2,3,5,6- tetrafluoro-1,4-phenylene or 2,2',3,3',5,5,6,6'-octafluoro-4,4'-biphenylene) in the presence of (B) a basic substance such as an alkali metallic fluoride in (A) an organic solvent sparingly soluble in water. (E) Water is then added to the resultant reactional product solution to separate an organic layer. Thereby, the compound of the formula is obtained.