To provide a device and method for adjusting antenna radiation power which automatically measure tag operation power by considering loss (attenuation) caused by cable length difference or the like, and thereafter automatically correcting the difference during actual operation to adjust the values of actual radiation power radiated on the air from respective antennas to the same level.

First, tag operation power is automatically measured considering loss (attenuation) caused by cable length difference or the like using a reference tag (black rectangle on the figure). When the cable length A (113) from an antenna port A of an RW 110 to an antenna A (111) is 5 m (attenuation=2.5 dB) and the cable length B (114) from an antenna port B of the RW 110 to an antenna B (112) is 10 m (attenuation=5 dB) in the measurement result thereof, the relationship of the respective port outputs of the RW 110 is set so that the values of actual radiation power radiated on the air coincide with each other by assuming B= A+(5-2.5)..[dBm], wherein A, B represent radiation power values [dBm] of the respective output ports.