A magnetic sensor device detecting a magnetic body (a, b, c) contained in a paper sheet transported on a transport path, the device comprising: a magnetic field generation unit which generates a bias magnetic field in an arbitrary direction on a plane perpendicular to a transport direction of the paper sheet on the transport path; a magnetoelectric conversion element (4; 41, 42, 43) which is provided in the bias magnetic field and between the transport path and the magnetic field generation unit and detects magnetic field intensity in a direction identical to a direction of the bias magnetic field; a correction unit which sets a reference point for representing an output value of the magnetoelectric conversion element by an offset corresponding to the bias magnetic field generated by the magnetic field generation unit; an amplification unit into which an output signal of the magnetoelectric conversion element and an output signal of the correction unit are input and which amplifies differential output of the output signal of the magnetoelectric conversion element and the output signal of the correction unit; a determination unit which determines a characteristic of the magnetic body contained in the paper sheet, according to an output value from the amplification unit; a feedback control unit which feeds back an output value from the amplification unit to input of the amplification unit so that an output value from the amplification unit becomes the reference point; and a switching means which switches between the feedback unit and the correction unit to be input to the amplification unit, wherein the output of the magnetoelectric conversion element changes linearly with respect to magnetic field intensity, and feedback time for the feedback control unit to make the output value from the amplification unit to become the reference point is longer than the delay time of the paper sheet to pass entirely through the magnetic sensor device. a magnetic field generation unit which generates a bias magnetic field in an arbitrary direction on a plane perpendicular to a transport direction of the paper sheet on the transport path; a magnetoelectric conversion element (4; 41, 42, 43) which is provided in the bias magnetic field and between the transport path and the magnetic field generation unit and detects magnetic field intensity in a direction identical to a direction of the bias magnetic field; a correction unit which sets a reference point for representing an output value of the magnetoelectric conversion element by an offset corresponding to the bias magnetic field generated by the magnetic field generation unit; an amplification unit into which an output signal of the magnetoelectric conversion element and an output signal of the correction unit are input and which amplifies differential output of the output signal of the magnetoelectric conversion element and the output signal of the correction unit; a determination unit which determines a characteristic of the magnetic body contained in the paper sheet, according to an output value from the amplification unit; a feedback control unit which feeds back an output value from the amplification unit to input of the amplification unit so that an output value from the amplification unit becomes the reference point; and a switching means which switches between the feedback unit and the correction unit to be input to the amplification unit, wherein the output of the magnetoelectric conversion element changes linearly with respect to magnetic field intensity, and feedback time for the feedback control unit to make the output value from the amplification unit to become the reference point is longer than the delay time of the paper sheet to pass entirely through the magnetic sensor device.