PURPOSE: To send ID information of a responder located only within a setting range from an interrogator by discriminating whether or not the responder is located within the setting range.

CONSTITUTION: A quasi-microwave sent from an antenna 14 is modulated by a responder 15, radiates again and is received by the antenna 14 again. A decoding circuit 16 decodes ID information of the responder 15 from the received reception signal and a reception electric field strength measurement circuit 18 measures an electric field strength of the radiating wave again to be received. A distance discrimination circuit 17 converts the electric field strength information into distance information and compares and discriminates reference distance information inputted and set by a reference distance setting circuit 21 in advance. When the distance discrimination circuit 17 discriminates the distance to be within a prescribed distance from the result of discrimination, a signal control section 19 sends the ID information decoded by the decoding circuit 16 from an external device connection terminal 20 by a signal control section 19.