A unit for measurement of absorbance using a microchip has a microchip with a continuous cavity, a sample chamber, a reagent chamber, a reagent mixing chamber and a chamber for measuring absorbance, which is arranged in a straight line in the area of the continuous cavity. The microchip is located in a chip holder which has a capillary part which is arranged such that the light used to measure absorbance is delivered through the capillary part to the chamber for measuring absorbance, the capillary part having a smaller opening diameter than the diameter of the cross section which is perpendicular to the optical axis of the chamber for measuring absorbance.