Theoretical and experimental investigations have been performed to clarify the mechanism of a self-excited chatter vibration induced in a paper-feed-roller system in an electrophotography copy machine. From results of the investigation, the following points were deduced: (1) The chatter vibration is induced only when a platen cover is pushed with a certain force and a paper does not exist between the roller and a platen glass, i.e., the roller slips against the platen glass. The experimental observation suggests that the vibration is a stick-slip vibration induced by the negative speed dependence of the friction coefficient. (2) Calculated results based on the present model qualitatively agreed with experimental observations, and several methods to suppress the vibration were proposed. (3) The paper-feed-roller system must be designed to be (a) high damping, (b) low negative speed dependence of the friction coefficient, (c) small press force, and (d) low arm angle. The resonance frequency of the platen cover must be designed to be away from the resonance frequency and its ultraharmonic ones of the roller system.