Abstract: The changing of the azimuth of directivity in which the azimuth of directivity of the antenna that achieves the maximum gain thereof is switched to the azimuths of tires is performed repeatedly for a plurality of cycles during a transmission duration of information. The output value of the antenna whose azimuth of directivity is controlled is sampled separately for each of the azimuths of directivity subjected to the switching. The sampled values are sorted into groups separately for each azimuth of directivity, and are accumulated separately for each group. The position of the tire shown by the azimuth of directivity of the group whose accumulated value is the largest is determined as being the position of the tire equipped with the transmitter that is transmitting the information.