A determining device, comprising: a voltage measuring instrument configured to measure phase voltages applied to power lines (La, Lb, Lc) configured to transmit alternating current power supplied from a three-phase alternating current power source; a current measuring instrument configured to measure phase currents flowing through the power lines (La, Lb, Lc), via current transformers (12a, 12b, 12c) placed on outer circumferences of the power lines (La, Lb, Lc); a phase acquirer configured to compute, for each phase, a phase of phase voltage and a phase of phase current from the phase voltages measured by the voltage measuring instrument and the phase currents measured by the current measuring instrument; a voltage/current verifier configured to compute, for each phase, a phase difference between phase voltage and phase current from the phase of phase voltage and the phase of phase current computed for each phase by the phase acquirer, and determine conformity if all of the computed phase differences are within a preset range; characterised in that it further comprises: a current/current verifier configured to compute phase differences between phase currents from the phase of phase current computed for each phase by the phase acquirer, and determine conformity if all of the computed phase differences are less than 180°; and a reporting device configured to report that the current transformers are incorrectly placed on the power line (La, Lb, Lc) unless both the voltage/current verifier and the current/current verifier determine conformity. a voltage measuring instrument configured to measure phase voltages applied to power lines (La, Lb, Lc) configured to transmit alternating current power supplied from a three-phase alternating current power source; a current measuring instrument configured to measure phase currents flowing through the power lines (La, Lb, Lc), via current transformers (12a, 12b, 12c) placed on outer circumferences of the power lines (La, Lb, Lc); a phase acquirer configured to compute, for each phase, a phase of phase voltage and a phase of phase current from the phase voltages measured by the voltage measuring instrument and the phase currents measured by the current measuring instrument; a voltage/current verifier configured to compute, for each phase, a phase difference between phase voltage and phase current from the phase of phase voltage and the phase of phase current computed for each phase by the phase acquirer, and determine conformity if all of the computed phase differences are within a preset range; characterised in that it further comprises: a current/current verifier configured to compute phase differences between phase currents from the phase of phase current computed for each phase by the phase acquirer, and determine conformity if all of the computed phase differences are less than 180°; and a reporting device configured to report that the current transformers are incorrectly placed on the power line (La, Lb, Lc) unless both the voltage/current verifier and the current/current verifier determine conformity. a current/current verifier configured to compute phase differences between phase currents from the phase of phase current computed for each phase by the phase acquirer, and determine conformity if all of the computed phase differences are less than 180°; and a reporting device configured to report that the current transformers are incorrectly placed on the power line (La, Lb, Lc) unless both the voltage/current verifier and the current/current verifier determine conformity.