Abstract: In polyphase AC load circuits, the current of each phase, which varies over a wide range, is led out to detect the unbalances of currents in the respective phases at arbitrarily adjustable predetermined unbalance factor by stationary-type device. The DC voltages (E.sub.U to E.sub.W) corresponding to the currents in the respective phases are divided into forward constant-voltage elements (D.sub.1 to D.sub.3) and passive voltage varying elements (VR.sub.1 to VR.sub.3). The voltages at the elements (VR.sub.1 to VR.sub.3) are defined as current comparison voltages (e.sub.1 to e.sub.3), and the voltages (E.sub.U to E.sub.W) in the respective phases are reduced by level adjusting circuits (LA.sub.1 to LA.sub.3) to reference level voltages (e.sub.U to e.sub.W). Both voltages (e.sub.U to e.sub.W) and (e.sub.1 to e.sub.3) are applied to the positive and negative terminals of the same elements (D.sub.4 to D.sub.6) to thus form an unbalance detecting circuit.