A mixed gas plasma of argon and hydrogen could be generated by irradiation of a microwave to perovskite-type oxide powders at atmospheric pressure. The results of electron and gas temperature measurements showed that this atmospheric pressure plasma is in a nonequilibrium. By using this plasma, we tried to remove surface scales, such as a metal oxide layer, on steel products. Consequently, the scales were removed with little damage in a short period of time, while it is difficult for the ordinary hydrogen reduction process to remove them. The plasma-treatment effect was advanced with increases in the microwave power and treatment time. Optical emission spectroscopy indicated that a large amount of hydrogen radicals existed in the plasma. It is considered that the existence of hydrogen radicals was effective in removing the scales rapidly.