1. A method comprising: outputting diagnostic light from a diagnostic light source section, the diagnostic light source section including a seed light source configured to output the seed light, and a silica optical fiber having a nonlinear optical coefficient of 20 W −1 km −1 or more to which the seed light is input, wherein the diagnostic light source section is configured to generate supercontinuum light as diagnostic light having a HE11 mode field pattern utilizing a nonlinear optical phenomenon within said silica optical fiber and output the diagnostic light; converging the diagnostic light and irradiating a measurement subject with the diagnostic light; acquiring object light generated at the measurement subject resulting from irradiation with the diagnostic light; receiving the object light and measuring a frequency spectrum of the object light; storing information of a frequency spectrum of a known substance; and calculating a correspondence between the frequency spectrum of the object light and the frequency spectrum of the known substance, and analyzing the measurement subject based on the correspondence, wherein a spatial density of an optical power is 1 mW/μm 2 or more at least at a part of the silica optical fiber of the diagnostic light source section, wherein the diagnostic light source section is configured to output the diagnostic light having an optical power of 1 μW/nm or more at least in a part of a spectrum band of wavelength from 0.8 to 3.0 μm, wherein the seed light source of the diagnostic light source section includes a pump pulse source, and in the diagnostic light source section, a spectrum of pump pulse light output from the pump pulse source is expanded utilizing the nonlinear optical phenomenon during propagation in the optical fiber so that the pump pulse light is output as the diagnostic light, wherein the step of acquiring object light further comprises acquiring, as the object light, diagnostic light having a loss generated at the measurement subject resulting from irradiation with the diagnostic light, wherein the frequency spectrum of the known substance contains a loss spectrum, and wherein the step of calculating the correspondence between the frequency spectrum of the object light and the frequency spectrum of the known substance further comprises calculating a correspondence between the frequency spectrum of the object light and the loss spectrum, and analyzing the measurement subject based on the correspondence.