Substrate treating device and substrate treating method

A substrate treating method for performing a predetermined treatment on substrates by immersing the substrates into a treating liquid that includes two or more kinds of chemical liquids containing at least phosphoric acid and pure water and is stored in a treating tank, the substrate treating method comprising:

• the substrates treating step of performing a predetermined treatment on a substrate by immersing the substrates into the treating liquid stored in the treating tank;
• a treating-liquid exchange step of exchanging the total treating liquid in the treating tank based on a life time of the treating liquid;
• a concentration detecting step of detecting a concentration of the pure water in the treating liquid supplied to the treating tank;
• a concentration control step of supplying pure water to the treating liquid in the treating tank such that the concentration becomes a predetermined target concentration of pure water, based on the concentration detected in the concentration detecting step;
• and
• a target-value change step of increasing the target concentration of pure water by changing a target value of a concentration based on a feedback control of the concentration of pure water, in a middle of a life time of the treating liquid, which is a period after a total liquid exchange is performed and before a total liquid exchange is performed again, and in a period when the predetermined treatment is performed.