Continuous Ni-P alloy plating without any effluence of waste plating solution was studied using a plating cell divided into cathode and anode compartments with a cation exchange membrane and introducing a dimensionally stable anode. Nickel and hydrogen ions reduced in the plating reaction could be replenished by those invading through the membrane from the anode compartment. The replenishment rate of these ions, which depends on their concentration ratio in the anolyte, was maintained at a suitable value by the reasonable distribution of the anodic current to the nickel anode and a dimensionally stable anode. In addition to replenishment through membrane, the continuous addition of phosphorous acid to the catholyte resulted in the constant composition of the plating solution and thus the constant composition of deposited films.