A magnetic circuit for a Faraday rotator capable of suppressing generation of irreversible demagnetization is provided. This magnetic circuit (100, 200, 300, 400, 500, 600, 700, 800, 900) for a Faraday rotator includes a first magnet (2, 202, 302, 702, 802), a second magnet (3, 203, 303, 703, 803) and a third magnet (4, 304, 604, 704, 804, 904), and a first high coercive force region (4b, 304b, 604b, 704b, 904b) is provided in the vicinity of at least the inner peripheral surface of a third through-hole (4a, 304a, 604a, 704a, 804a, 904a) of the third magnet.