Magnetic sensor device

A magnetic sensor device for sensing a sheet-like detection object magnetized by a magnetizing magnet that forms a magnetization magnetic field in a transport plane, magnitude of a magnetic field component parallel to the transport plane in the transport plane of the magnetization magnetic field being larger than or equal to a saturation magnetic field of a second magnetic body having a second coercivity larger than a first coercivity, the magnetic sensor device comprising:

• a bias magnet to form a bias magnetic field having a magnetic force direction of a center of a magnetic flux that intersects a plane of the detection object magnetized by the magnetizing magnet transported along the transport plane, wherein magnitude of a magnetic field component parallel to the plane of the detection object in the bias magnetic field occurring in the plane of the detection object is larger than the first coercivity, and is less than the second coercivity;
• and
• a magnetoresistive effect element disposed at the bias magnet and facing the plane of the detection object,
• wherein the bias magnetic field has, in a plane parallel to the transport plane, (i) a positive-direction component magnetic field that is directed in the same direction as a transport direction in which the detection object is transported, and (ii) a negative-direction component magnetic field that is directed opposite to the transport direction, and
• upon the detection object passing through the bias magnetic field, (i) for a first magnetic body having the first coercivity, a direction of magnetization of the first magnetic body reverses, and (ii) for the second magnetic body having the second coercivity, a direction of magnetization of the second magnetic body remains the same as that of magnetization of the second magnetic body by the magnetizing magnet.