A plasma processing apparatus includes a main container, one or more radio frequency antennas, a plurality of metal windows, and a plasma detector. The one or more radio frequency antennas are configured to generate inductively coupled plasma in a plasma generation region in the main container. The metal windows are disposed between the plasma generation region and the radio frequency antennas while being insulated from each other and from the main container. Further, a plasma detector is connected to each of the metal windows and configured to detect a plasma state.