PROBLEM TO BE SOLVED: To send biological information detected in a living body as optical signals generated by light in a near-infrared region and obtain signals with sufficient intensity and favorable S/N ratio.SOLUTION: A capsule endoscope 1 introduced into a living body M has functions to capture images and send the generated image data as optical signals generated by coherent light in a near-infrared region. The optical path of the near-infrared light representing the optical signals comprises: a first optical filter F1 for passing the coherent light therethrough a second optical filter F2 for passing near-infrared light in a frequency band compatible with the light from the endoscope 1 and a photodetector 21 having a light-receiving element group PD for receiving the light passed through the optical filters F1, F2. Each element of the light-receiving element group PD has a self-amplification function to generate a received-light quantity signal in which the intensity of the light entering the light-receiving element group PD is increased. The received-light quantity signal is input to a control unit 20 for restoration of the original image data.
