A copper-nickel alloy electroplating apparatus (1, 100), comprising: a cathode chamber (4, 104) in which a workpiece (5, 105) is to be placed; an anode chamber (6, 106); an anode (7, 107) placed in the anode chamber (6, 106); a diaphragm (14, 114) configured to provide an electrically conductive partition between the cathode chamber (4, 104) and the anode chamber (6, 106), and placed to separate the cathode chamber (4, 104) and the anode chamber (6, 106) from each other; a cathode chamber oxidation-reduction potential adjusting tank (8, 108) configured to adjust the oxidation-reduction potential of a plating liquid in the cathode chamber (4, 104); an anode chamber oxidation-reduction potential adjusting tank (10, 110) configured to adjust the oxidation-reduction potential of a plating liquid in the anode chamber (6, 106); a power supply unit configured to provide an electric current to flow between the workpiece (5, 105) and the anode (7, 107); characterised in that the apparatus further comprises: a cathode chamber electric potential measuring device (38, 138) configured to measure the oxidation-reduction potential of the plating liquid in the cathode chamber (4, 104); an anode chamber electric potential measuring device (42, 142) configured to measure the oxidation-reduction potential of the plating liquid in the anode chamber (6, 106); a cathode chamber adjusting agent addition device (40, 140) configured to add an oxidation-reduction potential adjusting agent to the cathode chamber oxidation-reduction potential adjusting tank (8, 108); an anode chamber adjusting agent addition device (44, 144) configured to add an oxidation-reduction potential adjusting agent to the anode chamber oxidation-reduction potential adjusting tank (10, 110); and a control unit (46, 146) configured to control the cathode chamber adjusting agent addition device (40, 140) and the anode chamber adjusting agent addition device (44, 144) on the basis of the oxidation-reduction potential measured by the cathode chamber electric potential measuring device (38, 138) and the oxidation-reduction potential measured by the anode chamber electric potential measuring device (42, 142). a cathode chamber (4, 104) in which a workpiece (5, 105) is to be placed; an anode chamber (6, 106); an anode (7, 107) placed in the anode chamber (6, 106); a diaphragm (14, 114) configured to provide an electrically conductive partition between the cathode chamber (4, 104) and the anode chamber (6, 106), and placed to separate the cathode chamber (4, 104) and the anode chamber (6, 106) from each other; a cathode chamber oxidation-reduction potential adjusting tank (8, 108) configured to adjust the oxidation-reduction potential of a plating liquid in the cathode chamber (4, 104); an anode chamber oxidation-reduction potential adjusting tank (10, 110) configured to adjust the oxidation-reduction potential of a plating liquid in the anode chamber (6, 106); a power supply unit configured to provide an electric current to flow between the workpiece (5, 105) and the anode (7, 107); characterised in that the apparatus further comprises: a cathode chamber electric potential measuring device (38, 138) configured to measure the oxidation-reduction potential of the plating liquid in the cathode chamber (4, 104); an anode chamber electric potential measuring device (42, 142) configured to measure the oxidation-reduction potential of the plating liquid in the anode chamber (6, 106); a cathode chamber adjusting agent addition device (40, 140) configured to add an oxidation-reduction potential adjusting agent to the cathode chamber oxidation-reduction potential adjusting tank (8, 108); an anode chamber adjusting agent addition device (44, 144) configured to add an oxidation-reduction potential adjusting agent to the anode chamber oxidation-reduction potential adjusting tank (10, 110); and a control unit (46, 146) configured to control the cathode chamber adjusting agent addition device (40, 140) and the anode chamber adjusting agent addition device (44, 144) on the basis of the oxidation-reduction potential measured by the cathode chamber electric potential measuring device (38, 138) and the oxidation-reduction potential measured by the anode chamber electric potential measuring device (42, 142). a cathode chamber electric potential measuring device (38, 138) configured to measure the oxidation-reduction potential of the plating liquid in the cathode chamber (4, 104); an anode chamber electric potential measuring device (42, 142) configured to measure the oxidation-reduction potential of the plating liquid in the anode chamber (6, 106); a cathode chamber adjusting agent addition device (40, 140) configured to add an oxidation-reduction potential adjusting agent to the cathode chamber oxidation-reduction potential adjusting tank (8, 108); an anode chamber adjusting agent addition device (44, 144) configured to add an oxidation-reduction potential adjusting agent to the anode chamber oxidation-reduction potential adjusting tank (10, 110); and a control unit (46, 146) configured to control the cathode chamber adjusting agent addition device (40, 140) and the anode chamber adjusting agent addition device (44, 144) on the basis of the oxidation-reduction potential measured by the cathode chamber electric potential measuring device (38, 138) and the oxidation-reduction potential measured by the anode chamber electric potential measuring device (42, 142).