A plasma processing apparatus includes a processing chamber, a conductive annular member, a microwave radiating mechanism and a plasma detector. The processing chamber has a ceiling plate with an opening. The conductive annular member is disposed at the opening while being insulated from the ceiling plate. The microwave radiating mechanism is disposed on the ceiling plate to be coaxial with a center of the conductive annular member and configured to radiate microwaves into the processing chamber. Further, a plasma detector is connected to the conductive annular member and configured to detect a state of generated plasma.