To provided an absorbance measuring unit for a microchip reduced in measurement errors, even if the absorbance of the microchip is measured, using a discharge lamp in a measuring light source.

The absorbance measuring unit using the microchip is composed of an absorbance measuring part, which comprises two bonded plate members, having cavity parts provided to the their bonded surfaces and is constituted so that the cavity parts are allowed to communicate with each other to linearly arrange an analyzing liquid inlet part, a bonded surfaces sump part and a reagent mixing part along one end surfaces of the plate members, the microchip equipped with the absorbance measuring part and a chip holder for incorporating the microchip. A light-introducing capillary part for making an optical axis coincide with that of the absorbance measuring part and having an opening diameter narrower than the cross-sectional diameter vertical to the optical axis of the absorbance measuring part is formed to the chip holder.