A magnetic-balance-system current sensor comprising: a magnetoresistive element, a resistance value of the magnetoresistive element being changed by application of an induction magnetic field generated by a measurement target current; magnetic cores disposed in the vicinity of the magnetoresistive element; a feedback coil disposed in the vicinity of the magnetoresistive element and generating a cancelling magnetic field, the cancelling magnetic field being a field cancelling out the induction magnetic field; and a magnetic-field detecting bridge circuit comprising the magnetoresistive element and having two outputs having a voltage difference therebetween, the voltage difference being generated in accordance with the induction magnetic field, the measurement target current being measured on the basis of a current flowing through the feedback coil when electricity is conducted through the feedback coil due to the voltage difference and the induction magnetic field and the cancelling magnetic field enter a state of equilibrium in which the induction magnetic field and the cancelling magnetic field cancel each other out, wherein the feedback coil, the magnetic cores, and the magnetic-field detecting bridge circuit are formed on a same substrate, and characterized in that the magnetic cores are two magnetic cores, the magnetoresistive element is disposed between the two magnetic cores, and the magnetoresistive element and the magnetic cores are formed in a same plane, and the feedback coil is disposed so that a direction orthogonal to a direction in which the measurement target current flows is an axial center direction of the feedback coil, and the magnetic cores are disposed so that an easy axis of magnetization of the magnetic cores is oriented in a direction that is orthogonal to the axial center direction of the feedback coil. a magnetoresistive element, a resistance value of the magnetoresistive element being changed by application of an induction magnetic field generated by a measurement target current; magnetic cores disposed in the vicinity of the magnetoresistive element; a feedback coil disposed in the vicinity of the magnetoresistive element and generating a cancelling magnetic field, the cancelling magnetic field being a field cancelling out the induction magnetic field; and a magnetic-field detecting bridge circuit comprising the magnetoresistive element and having two outputs having a voltage difference therebetween, the voltage difference being generated in accordance with the induction magnetic field, the measurement target current being measured on the basis of a current flowing through the feedback coil when electricity is conducted through the feedback coil due to the voltage difference and the induction magnetic field and the cancelling magnetic field enter a state of equilibrium in which the induction magnetic field and the cancelling magnetic field cancel each other out, wherein the feedback coil, the magnetic cores, and the magnetic-field detecting bridge circuit are formed on a same substrate, and characterized in that the magnetic cores are two magnetic cores, the magnetoresistive element is disposed between the two magnetic cores, and the magnetoresistive element and the magnetic cores are formed in a same plane, and the feedback coil is disposed so that a direction orthogonal to a direction in which the measurement target current flows is an axial center direction of the feedback coil, and the magnetic cores are disposed so that an easy axis of magnetization of the magnetic cores is oriented in a direction that is orthogonal to the axial center direction of the feedback coil.