To provide a method of treatment, by which a deposit adherent to, e.g., the inside of a reactor in a process for producing an -olefin oligomer is deactivated.

This method for deactivating the adhered deposit, comprising treating the adhered deposit accumulated on the inside of a reactor 10 and/or on the inside of a heat exchanger 16 for removing reaction heat in the reactor 10, on the production of an -olefin oligomer in the presence of a chromium-based catalyst in a solvent supplied to the reactor 10, is characterized by contacting the adhered deposit accumulated on the insides of the reactor 10 and/or the heat exchanger 16 with an electron-imparting compound (excluding the chromium-based catalyst existing in the reactor 10 on the production), after the operations of the reactor 10 and/or the heat exchanger 16 are stopped.