PURPOSE: To facilitate the measurement and adjustment of a phase difference while achieving a reduction in cost by eliminating the need for making a cable for connection accurately.

CONSTITUTION: In a phase difference measuring method, a reception level of a synthetic radio wave having a known phase radiated from transmitting antennas 1 and 2 is measured with a measuring device 5 beforehand and the reception level measured is compared with that of a radio wave having a phase difference unknown to measure a phase difference. In a phase difference adjusting method, a cable for opposite phase is inserted in a length equivalent to a half of the wavelength of input radio waves to be radiated from antennas 1 and 2 into one of cables 3 and 4 for connection to connect a wireless device and the antennas 1 and 2 and a cable for adjustment is inserted properly in a length less than the half wavelength of the input radio waves into either of the two cables 3 and 4 for connection to adjust the receiving level to the lowest. Then, the phase difference is adjusted by removing the cable for opposite phase.