To surely provide identification information from a wheel side device to a vehicle body side device, by improving a trigger device.

When a transmission instruction switch 106 is turned on and a trigger signal transmitted from another trigger device in a setting area is received, an informing part is informed about the possibility of a radio interference (S61). In this instance, no trigger signal is provided. When received identification information corresponds to identification information stored in a memory after transmission of the trigger signal , a count value is left intact (YES in S55). It is found that a wheel side device supplying the trigger signal has transmitted the identification information previously, and the trigger signal is not received by a desired wheel side device. An operator can again supply a trigger signal by changing a relative position, or the like.

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