A sheet guide includes a base on which a sheet is placed; a pinion which is rotatably provided on the base; a pair of racks which are extended in a sliding direction, which sandwich the pinion therebetween, which are engaged with the pinion to move opposite to each other in the sliding direction; a pair of positioning members which are provided at one end portions of the pair of racks respectively, and which come closer and move away from each other by movement of the pair of racks to position the sheet in the sliding direction; a sliding guide which is formed in the base and which guides the pair of racks in the sliding direction; and a pressing member which is provided on the base and which presses a portion of at least one rack of the racks to urge the at least one rack against the pinion.