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. The guide device also includes an urging member positioned between at least one of the rack members and a one of the slide guides facing the at least one of the rack members, in which the urging member is configured to urge the at least one of the rack members in the first direction when the side guides are at a predetermined position which is closer to the pinion than the first position which corresponds to a position at which the side guides are a maximum distance from the pinion.