A concentration measurement device that is provided with: at least one light source; a measurement cell that is for inserting a fluid to be measured; a brancher that is for making light from the light source branch into incident light that enters the measurement cell and non-incident light that does not enter the measurement cell; a transmitted-light detector that is for detecting transmitted light, which is incident light that has passed through the measurement cell; a non-incident-light detector that is for detecting the non-incident light; and a calculation unit that uses a detection signal from the non-incident-light detector to correct a detection signal from the transmitted-light detector.
