A process for, with use of a tray in which substrate receiving holes are provided and which has substrate support portions protruding from inner walls of the substrate receiving holes, placing the tray onto a tray support portion of a substrate stage and placing substrates onto the substrate holding portions, so that edge portions of the substrates projected out of end edges of the substrate holding portions and the substrate support portions are separated; a process for reducing pressure in a chamber and supplying a process gas thereto to fulfill plasma processing for the substrates; and a process for, with the tray and the substrates placed on the substrate stage, reducing the pressure in the chamber and supplying a process gas to fulfill plasma processing so that by-products stuck to edge portions of the substrates and the substrate support portions are removed.