Provided are a concentration-factor measurement device and method for accurately calculating a concentration factor of circulating water, and a method for measuring a water-quality index value of the circulating water. The concentration-factor measurement device includes a light-absorbance measurement unit (1) for measuring absorbance by irradiating each of cells (12A-12C) with light, and an electrode measurement unit (2) equipped with electrodes (16, 17) that are inserted into a water sample in a container (20). For each item of water quality, the device calculates a concentration factor on the basis of a measured value measured when the water sample (W) is the circulating water, and a measured value measured when the water sample (W) is makeup water. A plurality of concentration factors are calculated on the basis of the measured values for plural items of water quality. Hence an accurate concentration factor can be obtained.