A position detecting device (3) includes magnetic field detectors (6a to 6h) that serve to detect a magnetostatic field generated by a permanent magnet inside a test capsule (2), a reference sensor (7) that serves to detect positions of the magnetic field detectors (6a to 6h), fixing members (9a, 9b) that serve to fix the magnetic field detectors(6a to 6h) on a surface of a subject (1), and a position information deriving unit (10) that derives a position of the test capsule (2) inside the subject (1). Since the reference sensor (7) detects the positions of the magnetic field detectors (6a to 6h) before the position of the test capsule (2) is derived, even when the subject (1) moves or changes a posture, the position of the test capsule (2) can be detected with accuracy.
