A microchip includes fluid circuits therein, formed by uniting together at least a first substrate that is a transparent substrate and a second substrate having grooves provided at the substrate surface and/or through holes penetrating in a thickness direction. The fluid circuits include a liquid reagent receptacle unit to store a liquid reagent, a quantification unit to quantify the liquid reagent or specimen, and an overflow liquid storage unit connected to the quantification unit to store the liquid reagent or specimen overflowing from the quantification unit during quantification. There is also provided a method of using the microchip.