The image signal converting apparatus is used in a lighting unit. The lighting unit includes a liquid crystal panel modulating a red component light according to a red image signal, a liquid crystal panel modulating a green component light according to a green image signal, and a liquid crystal panel modulating a blue component light according to a blue image signal. Yellow component light is superimposed on any of the red component light, the green component light and the blue component light. The image signal converting apparatus includes a controlling unit which controls a superimposition amount of yellow component light based on a specific image signal among a red image signal, a green image signal, and a blue image signal. A color corresponding to the specific image signal has a hue adjacent to a hue corresponding to the yellow component light. The controlling unit controls a reduction amount of the specific image signal based on the superimposition amount.