PURPOSE: To provide a solution concentration detector of high accuracy and reliability, and capable of continuously controlling a concentration without generating clogging, etc.

CONSTITUTION: The solution concentration detector comprises a main body 10 for covering a solution surface 2 and forming a air-sealed space, and also for making up the correlation between concentrations in the liquid phase and in the gas phase, of a solute within the air-sealed space, a gas agitating means 20 for agitating the atmospheric gas 3 within the main body 10, and a gas concentration detecting means 30 for detecting the concentration in the atmospheric gas 3 agitated by the gas agitating means 20, and the concentration in the liquid phase is detected from the concentration in the gas phase detected by the gas concentration detecting means 30 on the basis of the correlation.