Provided is a copper-nickel alloy electroplating apparatus which is capable of stably forming a copper-nickel plated coating on a workpiece with a uniform composition and which enables a plating bath to be used for a long period. The present invention provides a copper-nickel alloy electroplating apparatus (1), comprising: a cathode chamber (4) in which a workpiece (5) is to be placed; an anode chamber (6); an anode (7) placed in the anode chamber; an electrically conductive diaphragm (14) placed to separate the cathode chamber and the anode chamber from each other; a cathode chamber oxidation-reduction potential adjusting tank (8) for adjusting the oxidation-reduction potential of a plating liquid in the cathode chamber; an anode chamber oxidation-reduction potential adjusting tank (10) for adjusting the oxidation-reduction potential of a plating liquid in the anode chamber; and a power supply unit (36) that provides an electric current to flow between the workpiece and the anode.