According to one embodiment, a plasma processing apparatus includes a chamber configured to maintain an atmosphere depressurized below atmospheric pressure, a gas supply part configured to supply a gas into the chamber, a placement part provided inside the chamber, and configured to place a processed product, a depressurization part configured to depressurize inside the chamber, a window provided in the chamber, and facing the placement part, a plasma generator provided outside the chamber and on a surface of the window on an opposite side to the placement part, and configured to generate plasma inside the chamber, an optical path changing part provided inside the window and having a surface tilted to a central axis of the chamber, and a detection part provided on a side surface side of the window, and facing the surface of the optical path changing part.