A measuring device includes a substrate disposed on a substrate support of a plasma processing apparatus, a transmission circuit, a transmitting antenna, a receiving antenna, a reception demodulation circuit, and a calculator which are provided in the substrate. The transmission circuit generates a microwave. The transmitting antenna transmits the microwave generated by the transmission circuit as a transmission wave. The receiving antenna receives a reflected wave of the transmission wave by plasma above the substrate support as at least one reception wave. The reception demodulation circuit generates a signal that reflects a thickness of a sheath between the substrate and the plasma, from the reception wave. The calculator obtains the thickness of the sheath from the signal generated by the reception demodulation circuit.