To efficiently and accurately execute a search job in an interrogator to make execution of a search function possible to search for a transponder having designated specific information.

When a worker searches for an RF tag having the designated specific information (ID number), a control circuit transmits an interrogation wave from a transmitting part, based on reception of a response signal from the RF tag by the receiving part, the information on all the RF tags existing in a communication area is read (S1). At first, the communication area is presumed to be comparatively wider. When the RF tag as a target exists in the communication area (S2:Y), its existence and the number of the RF tags existing in the communication area are displayed on a display part (S4). When a plurality of the RF tags exist, a transmitting output regulator circuit is controlled and the communication area is automatically reduced, and the reduction of the communication area is reported (S6). Further, a job (S1) to reread is iterated.