A system is for detecting a position of a device. The device is swallowed, passes through a subject, and includes a magnetic field generator generating a constant magnetic field. The system also includes a position transducer including a magnetic detector and a position processor. The magnetic detector is disposed at a fixed relative position to the subject, and detects an intensity of a constant magnetic field output from the magnetic field generator. The position processor calculates a position of the device in the subject based on the intensity detected by the magnetic field intensity detector.