A substrate treating apparatus for treating substrates with a treating solution having a mixture of a chemical and a diluent. The apparatus includes a treating tank for storing the treating solution, a heating device for heating the treating solution, a supply pipe for supplying a gas at a fixed flow rate, the supply pipe having a detecting end at a predetermined depth in the treating tank, a pressure detecting device for detecting a pressure in the supply pipe, a converting device for converting the pressure detected by the pressure detecting device into a voltage, a storage device for storing, as a reference voltage, a voltage received from the converting device when a reference liquid at a reference temperature is stored in the treating tank, and a computing device for deriving an actual specific gravity of the treating solution from the reference voltage stored in the storage device, and a treatment voltage received from the converting device when the treating solution stored in the treating tank has been heated to a treating temperature by the heating device.