If a trigger signal transmitted from another trigger unit in a set range is received after a transmission command switch has been turned on, an annunciation portion is advised of a likelihood of radio interference. In this case, no trigger signal is supplied. If identifier information that has been received corresponds to the identifier information stored in a memory after the transmission of a trigger signal, a count value remains untouched (YES in S55). It is apparent that identifier information was transmitted before from one of wheel-side units that has supplied a trigger signal, and that the trigger signal has not been received by a desired one of the wheel-side units. An operator can supply a trigger signal again through a change in relative position or the like.