NOVELTY - A substrate treating apparatus (1) comprises circulating line (3) including a treating tank (11) for storing a phosphoric acid aqueous solution, circulating pump (13) for feeding phosphoric acid aqueous solution, heater (15) for circulation for heating the phosphoric acid aqueous solution, filter (17) for filtering phosphoric acid aqueous solution, a branch pipe (31) branching from the circulating line between the heater for circulation and the filter for extracting the phosphoric acid aqueous solution from circulating line; and a concentration measuring station connected to branch pipe.    USE - As substrate treating apparatus (claimed), for treating semiconductor wafers, substrates for liquid crystal displays, substrates for plasma displays, field emission displays, optical displays, magnetic disks, magnetic optical disks, photomasks, or solar cells.    ADVANTAGE - The substrate treating apparatus improves the accuracy of measuring silicon concentration in the phosphoric acid aqueous solution.    DETAILED DESCRIPTION - A substrate treating apparatus (1) comprises circulating line (3) including a treating tank (11) for storing a phosphoric acid aqueous solution, circulating pump (13) for feeding phosphoric acid aqueous solution, heater (15) for circulation for heating the phosphoric acid aqueous solution, filter (17) for filtering phosphoric acid aqueous solution, where the circulating line causing phosphoric acid aqueous solution discharged from treating tank to flow in order of circulating pump, the heater for circulation and the filter, and returning phosphoric acid aqueous solution from the filter to the treating tank; a branch pipe (31) branching from the circulating line between the heater for circulation and the filter for extracting the phosphoric acid aqueous solution from circulating line; and a concentration measuring station connected to branch pipe for measuring silicon concentration in phosphoric acid aqueous solution by potentiometry.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of substrate treating apparatus.    Substrate treating apparatus (3) Circulating line (1)    Treating tank (11)    Circulating pump (13)    Heater (15)    Filter (17)    Pipeline (18)    Switch valve (19)    Inner tank (21)    Outer tank (23)    Temperature sensor (25)    Lifter (27)