Provided is a concentration measuring apparatus, which measures a concentration of a fluid under a high-pressure environment, such as an environment in which a supercritical fluid is provided. The concentration measuring apparatus includes: a concentration meter for measuring a concentration of a first fluid contained in a fluid in the measurement line; a sampling line for transferring a process fluid of a processing space in which a substrate is treated in a high-pressure environment to the measurement line; a control valve for opening and closing the sampling line; a fluid pressure regulator installed downstream the control valve in the sampling line and configured to adjust the passing fluid to a set pressure; and a decompression tank installed between the sampling line and the measurement line.