A structure for a plasma processing chamber which makes it possible to control the potential therein and simplify the construction of the plasma processing chamber. A gas-introducing showerhead 34 is disposed in the plasma processing chamber 10 including a container 11 having a process space S for receiving a semiconductor wafer W, and a susceptor 12 disposed in the container 11, for mounting the received semiconductor wafer W thereon. The susceptor 12 is connected to high-frequency power supplies 20 and 46. An electrode support 39 of the gas-introducing showerhead 34 is electrically grounded. An electrically floating top electrode plate 38 of the gas-introducing showerhead 34 is disposed between the electrode support 39 and the process space S. The top electrode plate 38 has a surface exposed to the process space S. An insulating film 48 is formed of a dielectric material and disposed between the electrode support 39 and the top electrode plate 38.