To reduce the cost of a device and prevent disturbance to a power supply system by providing a filter capacitor for increasing the primary side turns of a load current detector to absorb the over-compensating part of a tested active filter.

At the time of confirming the compensating performance of a tested active filter device 3 with larger compensating ability than the higher harmonic generating capacity of a testing device 5, the primary turns of a load current detector 31 of the filter device 3 is increased, and on the secondary side of the current detector 31, the higher harmonic quantity is made coincide with the compensating capacity of the filter device 3 to confirm compensating action. At this time, there is the difference between a compensating current IAF3 and an actual testing device load high harmonic current ILH of the filter device 3 and ILH<IAF3, so that an over-compensating part is going to flow out to the power supply 1 side from the filter device, but this can be absorbed by a filter capacitor 52. Load equipment 4 of relatively small capacity can therefore be provided constantly along with the testing device 5.