A medium processor enabling prevention of occurrence of jamming and capable of realizing simple positioning of a magnetic head, and a magnetic head enabling prevention of degradation of reading accuracy of magnetic information and adaptable to the medium processor. The medium processor (1) comprises a transport passage (14) for transporting an information recording medium (for example, a check (17)), a frame (15) forming the transport passage (14), and a magnetic head (11) for magnetizing the magnetic substance (for example, an MICR character (17a)) printed on the surface of the information recording medium. The magnetic head (11) is buried in the frame (15) with a part of the frame (15) intervening. The magnetic head (11) adaptable to such a medium processor (1) comprises a magnet (11a) magnetized with different magnetic poles along the direction of transportation of an information recording medium (for example, a check (17)), a yoke (11b) fitted onto the outer periphery surface of the magnet (11a) facing a passage (14) for transporting the information recording medium, and a slit (11d) so formed in the yoke (11b) that the longitudinal direction is generally perpendicular to the transportation direction. When at least one end of the slit (11d) is closed, the yoke (11b) is longer than the magnet (11a) in the transportation direction.