An experimental and analytical investigation has been performed on vibration induced in a driving mechanism of a photoconductor drum in a color laser printer. The mechanism consists of a flywheel supported by a dry journal bearing and driven through a belt and a pulley that is connected to a shaft of the photoreceptor drum. From the results of the investigation, the following characteristics were deduced: (1) Pulley runout induced a vertical harmonic pull force to the flywheel through the belt and caused the vibration of the flywheel. (2) When the pull force is large enough to support the flywheel, the shaft of which is always in contact with the stator ring of the dry bearing, the harmonic vertical forced vibration takes place by the harmonic vertical force due to the eccentricity of the pulley through the belt. (3) On the other hand whirling takes place when the pull force is low. The shaft vibrates in the air gap not only in vertical but also horizontal directions. (4) The driving mechanism must be constructed to have small runout of the pulley and sufficient static tension of the belt.