A wafer is subjected to a plasma process, using plasma generated while a process gas is fed into a process room, and a plus DC voltage is applied to an electrostatic chuck in order to attract and hold the wafer on the electrostatic chuck by an electrostatic force. A minus DC voltage is applied to the electrostatic chuck while nitrogen gas is fed into the process room in order to cause DC discharge after the processed wafer is separated from the electrostatic chuck and before a next wafer is attracted and held on the electrostatic chuck. By doing so, plus electric charge in the gas is attracted to the electrostatic chuck, so that the surface of the electrostatic chuck is charged with plus electric charge, thereby preventing its attracting function from being deteriorated.