The fog image shaped as the fiber-type paper dust on the background occurs under the combination of the monocomponent impression development with the cleaner-less system. Occurrence of fiber-shaped fog images was studied by using two types of printers. The microscope observation of the fiber-shaped fog image indicates that many toner particles stick to the fiber-type paper dust on the photoreceptor and there are many toner particles without paper dust on the paper. The occurrence condition of the fiber-shaped fog image is different between the two different processes. The fiber-shaped fog image has periodicity in with-rotation printer, while no periodicity in counter-rotation printer. The fiber-type paper dust is collected by the development roller and this collected paper dust causes the fiber-shaped fog images in counter-rotation printer. The toner pressure pushes the fiber-type paper dust with toner out of the development room and the fibershaped fog image occurs in counter-rotation printer. The fiber-type paper dust is not caught by the development roller and kept turning with rotation of the photoreceptor in with-rotation printer. The paper dust is corona charged with positive polarity after scorotron charging device. However, it should be charged negatively, if it can not be caught by development roller. The styrene-acrylic polymerized toner can be considered to makes the paper dust tribocharged negatively at the place where the photoreceptor rubs against the development roller in with-rotation printer.