An intra-subject position detection system includes a subject insertable device introduced into a subject, moving through the subject; and including a magnetic field generator that generates a magnetostatic field. The system also includes a position detecting device arranged outside the subject and obtaining position information of the subject insertable device inside the subject. The position detecting device also includes a magnetic field detector that is arranged on the subject at a time of use and detects a strength of the magnetostatic field output from the magnetic field generator; a reference sensor that derives a position of the magnetic field detector relative to a reference position on the subject; and a position deriving unit that derives a position of the subject insertable device inside the subject based on the magnetic field strength and the position of the magnetic field detector.