According to an aspect of the present invention, there is provided a belt unit including: a belt that is formed in an endless shape; a first roller that supports the belt from an inner side of the belt; a second roller that supports the belt from the inner side of the belt; regulation walls that are disposed on both sides of the second roller and that each includes a boss protruding outwardly, the boss having a tapered portion; and plate frames that are disposed on both sides of the belt and that each includes: a first groove portion that supports the first roller; and a second groove portion that supports the second roller, the second groove portion having a rounded edge formed to be run on by the boss through the tapered portion.