Abstract: A guide device includes a pair of side guides configured to guide a sheet in a first direction, and a sliding mechanism configured to support the side guides and to slide the side guides in a second direction perpendicular to the first direction between a first position and a second position. The guide device also includes a pinion rotatably positioned at the tray between the side guides, and a plurality of rack members, in which each of the rack members is connected to and extends from a corresponding one of the side guides in the second direction, and includes a rack which engages the pinion. In addition, each of the rack members is configured to move in the second direction in response to a sliding of the side guides. The guide device further includes a plurality of slide guides positioned at the tray, in which each of the slide guides is configured to guide the rack members when the rack member move in the second direction.