Abstract: A nullification device of the invention comprises a nullifying signal generator 15 for producing a nullifying signal, and a controller 14 for measuring a predetermined period of time and on/off-controlling a mode change switch 16 provided between the signal generator 15 and a transmitting antenna 18. On the other hand, an activation device comprises a generator 23 for producing an activating signal for the nullification device. The activation device and the nullification device have connection terminals a, b and a', b', respectively, where the devices are in contact with each other, with the nullification device placed in a accommodating recess in the activation device.