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 . <EOS>