To more simply specify a mounting position of a tire with a sensor unit based on an information signal from the sensor unit.

A signal Sd (detection value) according to phase difference of an information signal Si received between both receiving antennas 12a, 12b is detected through a phase difference detection part 21. The signal Sd is obtained by mixing received signals of the respective receiving antennas 12a, 12b. An on-vehicle control device 11 determines that a transmission source of the information signal Si is a sensor unit corresponding to which of a front wheel or a rear wheel based on the signal Sd. The on-vehicle control device 11 further determines which determined sensor unit of the front wheel or the rear wheel the transmission source of the information signal Si is based on acceleration information (acceleration of a tire) included in the information signal Si.