To provide an image forming apparatus and a paper conveying device, suppressing turning of paper by the rotation of a roller pair even when a loop is formed when switching back the paper.

The paper conveying device 70 includes positive/reverse rotation rollers 74A and 74B, a paper guide part 75, and a paper stop roller. The paper guide part 75 is arranged in the conveying direction downstream before the switch back of the positive/reverse rotation rollers 74A and 74B, and is configured to form a conveying path of the paper and to change a paper advancing height of the conveying path. The paper stop roller is arranged in the conveying direction downstream before the switch back of the paper guide part 75, stops the paper S before the switch back, and forms a loop Sr in the paper S between the paper guide part 75 and the paper stop roller. Then, the paper guide part 75 changes the paper advancing height of the conveying path 78 so as to be narrower after the paper S advances to the conveying path 78 than before advancing.