To provide a quaternary ammonium salt capable of separating simply and easily a catalyst for oxidation reaction from a product in a reaction solution after reaction, in a method for producing the oxidized product by oxidizing an organic compound with hydrogen peroxide in the presence of the catalyst for the oxidation reaction.

There is disclosed a quaternary ammonium salt expressed by formula (1), wherein R¹, R², R³ each independently represents a hydrocarbon group which may have a substituent; R4 represents a hydrogen atom or a hydrocarbon group which may have a substituent; X^- represents a counter anion; and n is an integer of 1-20.