A radio-frequency tag detecting system comprising: a plurality of first radio-frequency tag communication devices which are located at respective predetermined positions and which are arranged to transmit transmitted signals toward a radio-frequency tag and to receive a reply signal transmitted from the radio-frequency tag, for communication with the radio-frequency tag; and a second radio-frequency tag communication device which is arranged to transmit a transmitted signal toward said radio-frequency tag and to receive a reply signal transmitted from the radio-frequency tag, for detecting the radio-frequency tag located within areas of communication of said plurality of first radio-frequency tag communication devices, characterised in that: said first radio-frequency tag communication devices are stationary radio-frequency tag communication devices and said second radio-frequency tag communication device is a portable radio-frequency tag communication device; said portable second radio-frequency tag communication device is provided with an identification radio-frequency tag which makes it possible to detect the position of the portable second radio-frequency tag communication device by the plurality of stationary first radio-frequency tag communication devices; and in that: said radio-frequency tag detecting system further comprises a transmission control portion operable to control transmission of said transmitted signals such that said plurality of stationary first radio-frequency tag communication devices and said portable second radio-frequency tag communication device transmit said transmitted signals at respective different timings. a plurality of first radio-frequency tag communication devices which are located at respective predetermined positions and which are arranged to transmit transmitted signals toward a radio-frequency tag and to receive a reply signal transmitted from the radio-frequency tag, for communication with the radio-frequency tag; and a second radio-frequency tag communication device which is arranged to transmit a transmitted signal toward said radio-frequency tag and to receive a reply signal transmitted from the radio-frequency tag, for detecting the radio-frequency tag located within areas of communication of said plurality of first radio-frequency tag communication devices, characterised in that: said first radio-frequency tag communication devices are stationary radio-frequency tag communication devices and said second radio-frequency tag communication device is a portable radio-frequency tag communication device; said portable second radio-frequency tag communication device is provided with an identification radio-frequency tag which makes it possible to detect the position of the portable second radio-frequency tag communication device by the plurality of stationary first radio-frequency tag communication devices; and in that: said radio-frequency tag detecting system further comprises a transmission control portion operable to control transmission of said transmitted signals such that said plurality of stationary first radio-frequency tag communication devices and said portable second radio-frequency tag communication device transmit said transmitted signals at respective different timings. said radio-frequency tag detecting system further comprises a transmission control portion operable to control transmission of said transmitted signals such that said plurality of stationary first radio-frequency tag communication devices and said portable second radio-frequency tag communication device transmit said transmitted signals at respective different timings.