An instrument (10) for measuring three-dimensional motion in a living body comprises a plurality of magnetism generators (12.sub.i) fixed to one of at least two objects (44, 46) moving relatively in the living body, a plurality of magnetic field sensors (14.sub.j) fixed to the other object in order to perform noncontact detection of the magnetic field of each magnetism generator (12.sub.i), and a signal processing means (26) for calculating relative position and direction between each magnetism generator (12.sub.i) and each magnetic field sensor (14.sub.j) according to a magnetic field detected by each magnetic field sensor (14.sub.j). The number of magnetism generators (12.sub.i) and magnetic field sensors (14.sub.j) is at least five, respectively.