A light measuring device is disclosed which can take in light from different portions of a plane light source efficiently to carry out measurement. The light measuring device for measuring light from a plane light source includes a spatial division device configured to carry out operation for successively taking in light from different portions of the plane light source. An optical condensing device condenses the light from the different portions of the plane light source taken in by the operation of the spatial division device. A detector receives the light condensed by the optical condensing device and outputs a signal corresponding to the received light.