To improve workability and prevent erroneous connection of wiring at attaching an alternative gauge to a check terminal when exchanging a gauge.

A coupler 10 has coupler terminals 90 at the sites opposing each of a plurality of transformer terminals of a check terminal 50. When the alternative gauge is attached to the check terminal 50 at exchanging a gauge connected to the check terminal 50, one end of the coupler terminals 90 is connected to the transformer terminals 56, another end is connected to an alternative gauge terminal of an alternative gauge, and the transformer terminals 56 and the terminal of the alternative gauge side are electrically connected through the coupler terminals 90. Consequently, each of the plurality (all) of coupler terminals 90 of the coupler 10 can be simultaneously and collectively connected to the corresponding transformer terminals 56.