It is to enhance mass-productivity through quick adhesion of an optical element 11 to an optical module holder 14 using an ultraviolet curable adhesive 7. A pressure differential caused by heating the bake plate causes the elastomeric diaphragms to contact the wafer. There is also included providing a peak low frequency RF voltage value ) during plasma processing, the peak low frequency RF voltage ) value representing a peak value of a low frequency RF voltage supplied to the plasma processing chamber.