The antenna circuit 42 includes: an MI magnetic sensor having an electric property which changes in accordance with a magnetic field change; a radio-frequency signal generator S1 to apply a radio-frequency signal to the MI magnetic sensor Z1; an inverter 92 to invert the radio-frequency signal; an adder 94 to reduce the radio-frequency signal by adding the inverted signal with a received signal obtained by the MI magnetic sensor Z1; and detectors D1 and D2 to detect the received signal in which the radio-frequency signal has been reduced. Further, included are a resonant circuit 620 including a magnetic sensor circuit Z1 and a resonant element such as quartz to retrieve a magnetic field change of a resonant frequency from a detected magnetic field change; and a resistance R0. Further, the magnetic sensor circuit 610 includes a magnetoresistance element 612, a DC power source 611 and a resistance R1.