To provide a tap position determining device capable of easily determining the current position of a tap from outside a transformer without newly constructing a circuit, a mechanism, etc., in the transformer.

The tap position determining device includes a primary-side CT 14 measuring a primary-side current of the transformer, a secondary-side CT 16 measuring a secondary-side current of the transformer, a waveform acquisition unit 20 which acquires waveforms of the currents at a predetermined time measured by the primary-side CT 14 and secondary-side CT 16 at the same time, an arithmetic unit 25 which computes an average value of effective values of the waveforms acquired by the waveform acquisition unit 20, and a determining unit 28 which determines the tap position of the transformer from the ratio of the primary-side average value to the secondary-side average value computed by the arithmetic unit 25.