A method of producing a diagram for use in selecting wavelengths of light in optical polishing end point detection is provided. The method includes polishing a surface of a substrate having a film by a polishing pad; applying light to the surface of the substrate and receiving reflected light from the substrate during the polishing of the substrate; calculating relative reflectances of the reflected light at respective wavelengths; determining wavelengths of the reflected light which indicate a local maximum point and a local minimum point of the relative reflectances which vary with a polishing time; identifying a point of time when the wavelengths, indicating the local maximum point and the local minimum point, are determined; and plotting coordinates, specified by the wavelengths and the point of time corresponding to the wavelengths, onto a coordinate system having coordinate axes indicating wavelength of the light and polishing time.