Abstract: An in-vivo information extracting system comprises a tag device (1) embedded in a living body, a relay device (2) disposed outside the living body and near the tag device (1), and main transceiver (3) for collecting the in-vivo information extracted by the tag device (1) through the relay device (2). The tag device (1) has a rectifier circuit for rectifying an electromagnetic wave received from the relay device (2) and generating an operating power. By thus generating, inside the tag device, the operating power needed to drive the tag device (1) from the electromagnetic wave fed from outside by means of an RFID, any cell or battery in the tag device (1) is not needed, and the size of the tag device (1) is accordingly reduced. Therefore, the tag device can be used in a living body almost permanently.