A compact, lightweight, and easy-to-use ESR-CT apparatus including a magnetic field leak line (5-G line) which is capable of imaging a small animal, such as a mouse, within 15 minutes, and of observing a desired region with a spatial resolution of 1 mm or less. A permanent magnet system is introduced of the apparatus includes pole pieces having a predetermined area which are opposed to each other through a measured space, yokes combined with the pole pieces, and a permanent magnet inserted in series so that at least one magnetic pole plane intersects perpendicularly to the closed magnetic circuit for magnetic coupling with the yokes. This makes it possible to locate a gradient coil system and a field scanning coil system sufficiently apart from end faces of the pole pieces, and to downsize the field scanning coil system so that the gradient field system is movable.