A line head includes: a positive lens system having two lenses with positive refractive power; an image-side lens array in which the image-side lens of the two lenses is arrayed in a plural number in first and second directions; an object-side lens array in which the object-side lens of the two lenses is arrayed in a plural number in the first and second directions; a light emitter array in which a plurality of light-emitting elements are arrayed on an object side of the positive lens system for the one positive lens system; and an aperture plate that forms an aperture diaphragm disposed on the object side of the positive lens system so that an image side is telecentric or approximately telecentric. Assuming that the row number of lenses arrayed in the second direction of the image-side lens array is m, a gap between effective regions of the two image-side lenses adjacent to each other in the first direction is α, an image-side angle of aperture (half angle) of the positive lens system is θi, a width (full width) of a plurality of light-emitting element images in the first direction, which are images on image surfaces of the plurality of light-emitting elements arrayed for the one positive lens system, 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 the image surface is Si, the following conditions, f2≦(mWi−α)/(2θi), Wi≧2Siθi/(m−1)+α/(m−1), are satisfied.