A plasma processing apparatus includes a measurement unit that, while controlling a supply power to a heater such that a temperature of the heater becomes constant, measures the supply power to the heater in a non-ignition state where a plasma is not ignited and in a transient state where the supply power to the heater is reduced after the plasma is ignited. The plasma processing apparatus also includes a parameter calculator that calculates a thickness of a top plate by performing a fitting on a calculation model which includes the thickness of the top plate as a parameter and calculates the supply power in the transient state, using the measured supply power in the non-ignition state and the transition state.