To provide a method which can easily measure a concentration of a decomposition product in a plating solution without requiring expensive facilities.

A plating apparatus includes: plating parts 1 and 2 which immerse an article to be treated in the plating solution and form a thin film of a metal on the article to be treated by using the plating solution to which an additive necessary for forming the thin film and an analysis sample including metal ions having a lower standard electrode potential than the metal that forms the thin film have been added; an additive concentration detecting part 6 for detecting the concentration of the additive in the plating solution; an analysis sample concentration detecting part 7 for detecting the concentration of the analysis sample in the plating solution; and a decomposition product concentration calculating part 8 which recognizes a change of a concentration balance of the additive and the analysis sample with respect to the balance at the time when the plating solution has been supplied, on the basis of each detection result obtained in the additive concentration detecting part 6 and the analysis sample concentration detecting part 7, and calculates the concentration of the decomposition product of the additive in the plating solution.