Abstract: A plasma processing apparatus having a pair of electrodes which are installed alternately in parallel in a chamber and in which an object to be processed is placed at one electrode thereof, radiofrequency applying device for applying radiofrequency power between the pair of electrodes, cooling device for cooling the object, drying-gas introducing tube for supplying a drying gas into the chamber, and dropwise-condensation preventing member installed at a portion of the chamber so as to be in contact with the outer atmosphere. The apparatus can prevent dropwise condensation at the time of cooling and at the same time prevent the occurrence of radiofrequency leakage.