To provide a specific gravity measuring device, capable of accurately measuring the specific gravity of an etchant in real time by measuring the hydraulic pressure of the etchant, which is circulated in a circulation channel.

A part of an etchant circulated in a circulation channel flows into the retention part of a detecting tube 24. The hydraulic pressure of the etchant is detected by hydraulic pressure sensors 10 and 11 at the retention part, to detect its specific gravity from the detected hydraulic pressure difference. Since the specific gravity is detected from the hydraulic pressure of the etchant which is circulated as mentioned above, its spcific gravity can be detected with accuracy in real time, without being affected by bubbles even if they are produced in the etchant.