According to the present invention, the changing rate of the predetermined component concentration is determined, based on the difference between the predetermined component concentration in a chemical treating solution (a plating solution) which is analyzed this time and the predetermined component concentration analyzed last time, both measured by an analytical means, and the difference of each sampling time for analysis of each component concentration by the analytical means (140). Subsequently, the correction amount for the analyzed result of this time based on the changing rate obtained above and the elapsed time from the sampling point of time of the plating solution of this time for analysis of the plating solution by the analytical means to the current point of time (150), and then the analyzed result is corrected based on the resulting correction amount to compute or calculate the current concentration (160). As the result, the current concentration can be detected accurately regardless of the analyzing time of the plating solution by the analytical means.