Concentration measurement device and concentration measurement method

A concentration measurement device, comprising:

• a well that includes a side wall having a tubular shape and extending along a first direction and a bottom portion closing one end of the side wall having a tubular shape, and that stores a liquid, in which a sample is dissolved or suspended in water, in an internal region formed by the side wall and the bottom portion;
• a light irradiator that irradiates the liquid with a first light having a first wavelength and a second light having a second wavelength such that the first light and the second light pass through both the bottom portion of the well and a liquid surface of the liquid;
• a light detector that detects light intensities of the first light and the second light that have passed through the liquid;
• and
• an arithmetic processor that calculates a concentration of the sample in the liquid based on an optical path length of the first light in the liquid, which is calculated from a first absorbance that is an absorbance of the liquid for the first light, and a second absorbance that is an absorbance of the liquid for the second light,
• wherein an absorbance of the sample at the first wavelength is 0.005 or less, and an absorbance of water at the first wavelength is 0.2 or more,
• an absorbance of the sample at the second wavelength is 0.05 or more, and an absorbance of water at the second wavelength is 0.005 or less, and
• the light detector detects the light intensities of the first light and the second light that have passed through a center of the internal region or a position spaced apart from the center of the internal region by a distance of ¼ or less of an inner diameter of the side wall when viewed in the first direction.