In spectrum measuring equipment the light to be measured is separated by a double-image polarizing element into two polarized wave components whose planes of polarization cross each other perpendicular and which have different optical axes. The two polarized wave components are applied to a dispersing element so that their planes of polarization intersect the direction of light separation at .+-.45.degree. thereto, respectively. The sum of optical powers of the two polarized wave components separated by the dispersing element is measured by a photodetector, so that spectrum measurement independent of the polarization of the light to be measured can be achieved.