A system for detecting a position in a subject, comprising a body-insertable capsule device (2; 25) that is adapted to the introduced into the subject and to travel in the subject, and a position transducer (3; 26) that is disposed on the outside of the subject and is adapted to obtain information of a position of the body-insertable device (2; 25) in the subject, wherein the body-insertable device (2; 25) has a magnetic field generator that is adapted to output a constant magnetic field to the outside of the subject, and the position transducer (3; 26) includes a magnetic field detector (6a to 6h) that is adapted to sense magnetic fields; a magnetic field extractor that is adapted to eliminate a noise magnetic field component from the magnetic fields sensed by the magnetic field detector (6a to 6h) and to extract a constant magnetic field output from the magnetic field generator; a position information processor (10; 27) that is adapted to derive information of the position of the body-insertable device (2; 25) in the subject based on the intensity of the constant magnetic field extracted by the magnetic field extractor; characterized by further comprising: a noise magnetic field component storage unit that is adapted to store a direction and intensity of a noise magnetic field component sensed by the magnetic field detector (6a to 6h); a vertical direction sensor that is adapted to sense a correspondence relation between a vertical direction and a coordinate system in the magnetic field detector (6a to 6h); and a horizontal in-plane azimuth sensor that is adapted to sense a correspondence relation between an azimuth in a reference horizontal plane and a coordinate system in the magnetic field detector (6a to 6h), wherein the magnetic field extractor is adapted to extract a constant magnetic field output from the magnetic field generator by deriving a difference value between the magnetic field sensed by the magnetic field detector (6a to 6h) and the noise magnetic field component stored in the noise magnetic field component storage unit, the magnetic field extractor is adapted to derive the difference value in each azimuth between the magnetic field sensed by the magnetic field detector (6a to 6h) and the noise magnetic field component stored in the noise magnetic field component storage unit based on a correspondence relation sensed by the vertical direction sensor and the horizontal in-plane azimuth sensor to eliminate the noise magnetic field component, and the noise magnetic field component storage unit is adapted to store a noise magnetic field component including at least an earth magnetic component existing in a region in which the capsule is introduced into the subject as the noise magnetic field component. a magnetic field detector (6a to 6h) that is adapted to sense magnetic fields; a magnetic field extractor that is adapted to eliminate a noise magnetic field component from the magnetic fields sensed by the magnetic field detector (6a to 6h) and to extract a constant magnetic field output from the magnetic field generator; a position information processor (10; 27) that is adapted to derive information of the position of the body-insertable device (2; 25) in the subject based on the intensity of the constant magnetic field extracted by the magnetic field extractor; a noise magnetic field component storage unit that is adapted to store a direction and intensity of a noise magnetic field component sensed by the magnetic field detector (6a to 6h); a vertical direction sensor that is adapted to sense a correspondence relation between a vertical direction and a coordinate system in the magnetic field detector (6a to 6h); and a horizontal in-plane azimuth sensor that is adapted to sense a correspondence relation between an azimuth in a reference horizontal plane and a coordinate system in the magnetic field detector (6a to 6h), wherein