PURPOSE: To form an alloy plating layer with little variation of the alloy compsn. by calculating the decreased amt. of each ion of metal element based on the electric current applied on the plating liquid for plating and replenishing the plating liquid by the amt. accordant with the decreased amt. by dropping the liquid under specified conditions.

CONSTITUTION: When the alloy plating layer is continuously formed by applying a current on a plating liquid prepared by dissolving at least two kinds of metals, the quantity of electricity is calculated as the product of the current applied on the plating liquid and a specified unit time to apply the current, and then the decreased amt. of the ion of each metal element is calculated according to the quantity of electricity. Then a replenisher plating liquid containing metals according to the decreased amt. of ions calculated is continuously dropped in the next period of the specified unit time to the plating liquid by a certain amt. in such a manner that the replenishing time is ≥1/5 as long as the specified unit time. Thereby, the uniform compsn. is obtd. for the initial layer and the plating layer formed after a certain period is passed from the time when the initial plating layer is formed.