To miniaturize a transformer by winding a secondary winding to a secondary core so as to induce a secondary current which generates a magnetic field in the opposite direction to offset a magnetic flux generated in a magnetic core by a primary current supplied to a primary winding, and winding a part of the winding to a primary divided core.

The magnetic transformer 1 for a current sensor is provided with a first, a second input terminals 2, 3 to which a measurement current is input, a primary winding 5 wound to primary divided cores 4, 4 formed of material of a high magnetic permeability in a U shape, a secondary winding 9 wound to secondary divided cores 7, 7 formed of material of a high magnetic permeability in a U shape and forming a magnetic core 8, and a first, a second output terminals 10, 11 connected to ends of the winding 9. End parts of the cores 7 are arranged to face end parts of the cores 4. The cores 7 are magnetically coupled to the cores 4 with slits 6 provided between the confronting end parts. A magnetic sensor is inserted in either one of the slits 6 to detect a magnetic flux density in the magnetic core 8.