To provide a cleaning method effective for reducing contents of unreacted phenols, unreacted alcohols and a metal salt remaining in a phosphonitrilic acid ester by a cleaning process after production of a phosphonitrilic acid ester by reacting phosphonitrile dichloride with an alkali metal arylate and/or an alkali metal alcoholate.

In carrying out a cleaning process for removing unreacted phenols and/or alcohols from a reaction solution obtained by reacting a cyclic and/or chain phosphonitrile dichloride with an alkali metal arylate and/or an alkali metal alcoholate, (1) the amount of the unreacted phenols and/or the alcohols in the reaction solution to be supplied to the cleaning process is controlled, (2) a temperature to carry out the cleaning process is controlled, (3) the concentration of an aqueous solution of a metal hydroxide to be used in an alkali water cleaning process is regulated, (4) pH in an acid cleaning process is controlled and (5) a rinsing and cleaning process is performed.