In a tag reader/writer 10 according to the present invention, first, a temporary ID receiver 11 receives a temporary ID unit of temporary IDs of RFID tags 41-44, and 47 detected by another tag reader/writer 20, and stores them into a temporary ID storage DB 12. When the tag reader/writer 10 detects RFID tags 41, 43, 45, 46, and 48 within a communication area of its own, a temporary ID acquirer 13 thereof acquires temporary IDs of those RFID tags and stores them into the temporary ID storage DB 12. Thereafter, the tag reader/writer 10 makes a temporary ID comparator 14 compare temporary ID units stored, to specify a temporary ID corresponding to a difference between them, based on the result of the comparison. When the temporary ID is a decrease, it is determined to be a lost article, and a message of the lost article is sent. When the temporary ID is an increase, it is determined to be a newly purchased article, and related advertisement information is provided.