The method for measuring the concentration of the measuring object uses a sensor chip comprising an optical waveguide layer and an antibody immobilized layer formed on the surface of the optical waveguide layer, which comprises immobilizing the measuring object and an enzyme-labeled antibody labeled with a labeling enzyme on the antibody immobilized layer of the sensor chip having an immobilized antibody, producing a color-developing and precipitating enzyme reaction product by allowing to react a coloring reagent with the labeling enzyme on the antibody immobilized layer to precipitate the enzyme reaction product on the antibody immobilized layer, allowing to totally reflect a light impinged on the sensor chip from the outside at an interface between the optical waveguide layer and the antibody immobilized layer, and observing to a physical value of the totally reflected light.