The present invention relates to a method for purifying a compound represented by the formula (I), the method including treating a mixture containing the compound represented by the formula (I) and a compound represented by the formula (II) with an aqueous solution of a basic compound. where, in the formula (I), R1 represents a hydrogen atom or a chlorine atom; in the formula (II), R2, R3 and R4 are the same or different from each other and represent a hydrogen, a chlorine atom or a fluorine atom independently, at least one of R3 and R4 represent a chlorine atom or a fluorine atom, and, in the case where R3 represents a chlorine atom, R4 represents a chlorine atom or a fluorine atom.