PROBLEM TO BE SOLVED: To correctly measure an AC loss by applying a "cancellation method" to a three-phase superconducting transformer in which start connection is performed on at least one of primary-side and secondary-side windings.SOLUTION: In a three-phase superconducting transformer performing Y connection on both a primary-side winding 10 and a secondary-side winding 11 immersed in a liquid coolant and accommodated in an extra-low-temperature container, a power source 4, a shunt resistor 5 of a measurement circuit and a cancel coil 6 are connected to external terminals of two phase windings selected out of the primary-side winding 10, and external terminals of phase windings of the secondary-side winding 11 corresponding to the phase windings are short-circuited by a short-circuit line 18. In such a state, single-phase power is supplied between the two phase windings, and an AC loss is measured according to a cancellation method. Specifically, the AC loss is measured while combining external terminals o
