A compensation ring 31 disposed to surround a periphery of a wafer W on a susceptor 30 is concentrically divided into an inside first compensation ring member 32 and an outside second compensation ring member 33. A width of a first compensation ring member 32 is made such thin as one to three times mean free path of treatment gas molecules, thereby suppressing heat transfer between a susceptor 30 and a second compensation ring member 33. A base of a second compensation ring member, through a layer of conductive silicone rubber 34, is made to come into an intimate contact with an upper surface of a susceptor 30, thus helping to cool.