an absorption photometer for measuring the absorption by conducting the light to a thin flow cell in which a liquid sample flows , wherein the sample light for measuring the absorption peak is superimposed on the reference light selected from the transparent range of the liquid and the absorbance is detected by separating the sample light and reference light after transmission of the flow cell . changes in the light path conditions can be monitored accurately , and therefore high - accuracy measurement immune to noises is made possible even using an elongated flow cell .