PURPOSE: To obtain the title compound in high yield under a mild condition at high reaction rate without requiring addition of alkali by reacting a specific phosphonium salt with a metallic salt of dialkyl phosphate.

CONSTITUTION: (A) A phosphonium salt (e.g., tetra-n-butylphosphonium chloride) shown by formula I (R1-R4 are H, alkyl, hydroxyalkyl, allyl, benzyl or aryl; X is halogen) is reacted with (B) a metallic salt of dialkyl phosphate (e.g., sodium o,o-diethyl dithiophosphate) shown by formula II (R5 is 1-18C alkyl or aryl; Y is O or S; M is n-valent metallic ion or ammonium ion; n is 1 or 2) to give the aimed compound shown by formula III.