An image forming apparatus and a belt unit are provided. The image forming apparatus or the belt unit includes: an endless belt; a driving roller; a driven roller; a tension generating member which generates a tension at the belt; a displacement member which is provided to be displaced in a displacement direction parallel to the axial direction of the driven roller together with the belt when the belt is moved in a moving direction along the displacement direction; and an axial force converting unit which converts a force of displacing the displacement member into a force which is directed in a direction intersecting the displacement direction and causes the tension generated at the belt to change such that a ratio of a tension generated on a trailing side in the moving direction of the belt to a tension generated on a leading side in the moving direction is decreased.