Disclosed is a A reactive sputtering apparatus for a bias sputtering method of applying a bias voltage to a supporting substrate in formation of a film of a metal compound on the supporting substrate according to a reactive sputtering method; which comprises a supporting substrate conveyor unit and a cathode that includes a target provided to face the supporting substrate conveyor unit, and wherein a supporting substrate is conveyed between the supporting substrate conveyor unit and the target for formation of a metal compound on the supporting substrate, magnets are provided adjacent to the supporting substrate conveyor unit on the side thereof opposite to the supporting substrate in such that the magnetic field is closed and the continuing tunnel part of parallel or nearly parallel arched magnetic force lines forms an oval or a polygon, on the supporting substrate, the magnets each having a first magnetic pole of an S pole or an N pole and a second magnetic pole opposite to the first magnetic pole, the second magnetic pole surrounding the first magnetic pole and, in film formation on the supporting substrate, the supporting substrate is conveyed on the same plane as that of the tunnel part and in the direction nearly perpendicular to the tunnel part.