To make tag device usable being kept in an organism for a long time without forcing inconvenience to the organism such as a human being or an animal or giving a feeling of big resistance or pain.

This in-vivo information detecting system is provided with the tag device 1 embedded in the organism body; the relay device 2 installed outside the body near the tag device 1 embedded in the body; and a main transmitter-receiver 3 for acquiring in-vivo information detected by the tag device 1, through the relay device 2. The tag device 1 is provided with a rectifying circuit for rectifying electromagnetic wave received from the relay device 2, to generate operating power. The operating power required to drive the tag device 1 is generated inside a tag on the basis of the electromagnetic wave supplied from the outside, using a means such as RFID. There is therefore no need to provide the tag device 1 with a battery or the like, and the tag device 1 can be miniaturized by that portion and semipermanently used inside the body.