PURPOSE: To measure copper concn. in soln. accurately and continuously independently of the pH value of soln. to be measured, by controlling intensity of incident light on an optical cell due to the pH, and detecting intensity of light after passing the optical cell with photodelector.

CONSTITUTION: Light emitted from a source 1 such as tungsten lamp is made to monochromatic light having a specified wavelength by a filter 2, it is converged by a lens 3 and made incident on electroless copper plating liquid being soln. to be measured in the optical cell 4. Light passed through the cell 4 is made incident on a photodetector tube 5 such as photomultiplier tube to measure the intensity. Further the pH value of the above described soln. is measured by a pH measuring electrode 6, the output is converted to low impedance by an impedance converter 7, then the source 1 is controlled through optical source control circuit combined with a nonlinear feed back circuit 9. In this way, fluctuation of the pH value is corrected, copper concn. in plating liquid is continuously and accurately detected and plating liquid can be stably and accurately controlled.