<p>In the present invention, a loop antenna (35) that generates an interfering magnetic field is provided to the vicinity of an insertion opening (22) to which a magnetic card (21) is inserted, and an interfering magnetic field is caused to constantly act anteriorly and posteriorly to the insertion opening (22), thus preventing illicit acquisition of magnetic data by means of a lead connected to a magnetic head (34) for detecting a magnetic stripe or illicit acquisition of magnetic data by means of an illicit reading device connected anteriorly to the insertion opening (22). The interfering magnetic field also reaches to the magnetic head (34), but it is possible to detect the presence of magnetic data at the magnetic stripe of a magnetic card (21) by using the fact that the pattern of the detection signal before detecting the magnetic stripe and the pattern of the detection signal after detecting the magnetic stripe differ.</p>
