A line head includes a positive lens system having two lenses, image-side and object-side lens arrays, a light emitter array on an object side of the positive lens system, and an aperture plate that forms an aperture diaphragm. Where the row number of lenses arrayed in a second direction is ‘m’, a gap between effective regions of two image-side lenses adjacent to each other in a first direction is ‘α’, an image-side angle of aperture is ‘θi’, a width of light-emitting elements images in the first direction is ‘Wi’, a focal length of the image-side lens is ‘f2’ and a distance from an image-side principal plane of the image-side lens to an image surface is ‘Si’, the following conditions are satisfied: f2≦(mWi−α)/(2θi), Wi≧2Siθi/(m−1)+α/(m−1).