To provide a sensor unit arranged in the vicinity of the surface of a sheet-like signal transmission apparatus and transmitting results of various measurements to the signal transmission apparatus.

An electromagnetic field is propagated between first and second conductor parts 111, 121 in the sheet-like signal transmission apparatus, which has a region 141 in the vicinity of the surface thereof wherein the electromagnetic field infiltrates, sensor tags 942 each comprising an interface unit connected with a communication unit allocated with an identification code are arranged apart from the signal transmission apparatus, when some of the sensor tags 942 belong to the region 141 wherein the electromagnetic field infiltrates, the sensor tags 942 are communicable with the signal transmission apparatus, and when some of the sensor tags 942 belong to the region 141 wherein the electromagnetic field infiltrates, the sensor tags 942 transmit the identification code to the signal transmission apparatus so as to detect that the sensor tags 942 belong to the region wherein the electromagnetic field infiltrates.