Liquid concentration detecting method and apparatus

The present invention provides a liquid concentration detecting method and a liquid concentration detecting apparatus in which light beams of at least two different wavelength bands having a central wavelength within a range of from 1.4 to 2.05 mum are irradiated on to a solution, and concentrations of at least two constituents contained in the solution are detected by detecting the amount of light transmitting through the solution relative to the light beams of each wavelength band. The present invention permits inline real-time detection at a high accuracy of concentrations of a plurality of constituents contained in a chemical solution used in a semiconductor manufacturing process or a liquid crystal substrate manufacturing process. Further, according to the present invention, it is possible to high-accuracy and high-reliability detection of the liquid concentration with a simple configuration.