Provided is a concentration measuring device which includes: a main pipe through which a fluid whose concentration is to be measured flows; a flow cell having a fluid passage and a light passage formed to pass through the fluid passage; a spectrometer capable of measuring absorbance for each wavelength of the source light transmitted to the flow cell and the received light receiving from the flow cell; and an optical cable connecting the flow cell and the spectrometer with each other, wherein the flow cell is separated from the main pipe to be provided separately, and a fluid pipe is connected to the flow cell, so that the fluid flows from the main pipe to the flow cell through the fluid pipe using a pitot tube.