Color reagent, concentration measuring kit, method of concentration measuring and sensor chip for use therein

A method of concentration measuring in which the concentration of measuring subject substance in minute amounts of analyte can be measured in a short period of time and in which even when the amount of analyte measured is inaccurate, the concentration of measuring subject substance can be accurately measured. There is provided a method of measuring the concentration of measuring subject substance with the use of a sensor chip having a light guide layer and, disposed on the surface thereof, an antibody immobilization layer, which method comprises dropping an analyte solution and an enzyme-labeled antibody solution onto the antibody immobilization layer of the sensor chip to thereby carry out an antigen antibody reaction so as to fix the analyte; subsequently dropping a color reagent solution to thereby form coloring and precipitating enzymic reaction products so that the enzymic reaction products are deposited in the antibody immobilization layer; causing light incident on the sensor chip from outside to be wholly reflected by the antibody immobilization layer; and observing any change of physical quantity of the wholly reflected light.