To noncontactly detect a physical change of a measuring object, and to noncontactly detect also a physical change in an inside of the measuring object.

A sensor means for changing an electric characteristic in response to a physical variation of the measuring object is constituted of an oscillator 30 and a thermistor 31, the sensor means is attached to a tag main body 20, a clock signal generating means 22 for detecting a frequency of the oscillation circuit 30, an oscillation period counting means 24 and a counted result holding means 25 are provided in the tag main body 20, and a physical quantity of the measuring object detected by the thermistor 31 is detected as an oscillation frequency of the oscillator 30 to be transmitted to a reader/writer unit.