An image forming apparatus including a guide unit including: a length-adjusting assembly including a length-adjusting rack part, which is adjusted according to a length of printing medium; a width-adjusting assembly including a width-adjusting rack part, which is adjusted according to a width of printing medium; and a pinion assembly, which interlocks the width-adjusting assembly interlock with the length-adjusting assembly. The pinion assembly includes a first pinion part, which engages with the length-adjusting rack, and a second pinion part, which engages with the width-adjusting rack and rotates together with the first pinion part. The first pinion part and the second pinion part have a stepped shape.