To rapidly measure a dissolution amt. of metal with high accuracy even if a change of the concn. of metal ions is small before and after dissolution by calculating the dissolution amt. of metal ions dissolved by a metal dissolving device on the basis of a change of a pH value before and after dissolution.

When a plating soln. is supplied to the dissolving tank 12 of the metal dissolving apparatus provided to an electroplating apparatus main body 10 by a pump 14, it comes into contact with metal nickel housed in the dissolving tank 12 to dissolve nickel. The plating soln. containing the dissolved nickel ions is refluxed to the apparatus main body 10 and nickel ions are supplied to a plating bath. At this time, nickel ion concn. [Ni2+] before dissolution is measured by a densitometer 18 and pHi before dissolution is measured by an inlet side pH meter 20 and pHo after dissolution is measured by an outlet side pH meter 22 and these measured results are outputted to a control unit 24. In the control unit 24, the nickel ion concn. (dissolution amt.) [Ni2+]o after dissolution is calculated on the basis of the measured results. As a result, the concn. of metal ions in the plating bath can be controlled with high accuracy.