To reduce the surface states of GaAs and related semiconductors which originate from native oxides on a surface, we developed a simple surface treatment method in which the surface oxides could be physically sputtered by Ar plasma irradiation in an electron cyclotron resonance chemical vapor deposition (ECR-CVD) apparatus. In the experiment of Ar irradiation of a GaAs surface, we were able to determine the optimum irradiation time at which the native-oxide-related surface states were almost removed without damaging the irradiated surface. Then, we applied this method to an actual 0.98-µm semiconductor laser in which catastrophic optical damage (COD) failure occurred due to the surface states of the output facet. By irradiating Ar plasma to the output facet of the laser and removing the oxide-related surface states there, tolerance to COD was remarkably improved compared with that of a conventional non-Ar-irradiated one.