Signals are sent by radio waves from a single or multiple antennas provided in an external unit located outside of a living body to a capsule inserted into a living body to obtain living body information. The signals are received by the antenna on the capsule side and the radio wave strength data is obtained. Based on the radio wave strength data, living body information is sent from the capsule side to the external unit by using a transmission power suitable for the transmission. The radio wave strength data is sent to the external unit side, and the external unit side receives living body information by selecting and using an antenna, which can receive data with the highest radio wave strength.