A signal processing circuit of an external device includes a CPU and a memory which are not shown. A program for estimating at least one of the position and orientation of a capsule endoscope on the basis of strength signals received through respective antennas is installed in the signal processing circuit. A single-core coil to generate a magnetic field is arranged in the capsule endoscope. The generated magnetic field is detected by a plurality of coils arranged outside a body, whereby a distance that the capsule endoscope has traveled can be obtained with accuracy. This arrangement controls image-capture timing to reliably capture images necessary for a diagnosis and prevent unnecessary image capture.