PURPOSE: To obtain a method by which the distance between a transmitter and receiver can be accurately measured even when the transmitting power is unknown.

CONSTITUTION: A plurality of kinds of radio waves having different frequencies in a millimeter wave band and transmitted with the same power or radio waves having modulated spectra which are fixed in unit spectrum density in a prescribed frequency range in a millimeter wave band are received and the distance between a transmitter 5 and receiver 6 is calculated based on the frequency characteristic of attenuated amount of the radio waves by the air in the millimeter wave band from the receiving power or unit spectrum density at the different frequencies. Therefore, the distance between the transmitter 5 and receiver 6 can be measured even when the transmitting power is not recognized on the receiving side and, in addition, the measurement accuracy is not affected by the fluctuation of the transmitting power.