[Problem] When a nonreciprocal device operating at 100 GHz to 300 GHz is to be realized by using a conventional magnetic material of garnet-type ferrite or spinel-type ferrite, a huge permanent magnet is required and, therefore, it is very difficult to achieve a millimeter-wave band nonreciprocal device for practical use.[Solving means] To solve this problem, there is provided a millimeter-wave band nonreciprocal device composed of a magnetic material represented by a formula ε-MxFe2-xO3 (0