1.  Field of the Invention
The present invention relates to an image forming apparatus which records an image formed in an image forming unit onto a recording medium, including a belt which rotates while bearing toner images. 
2.  Description of the Related Art
Recently, a so-called intermediate transfer method has been known as a method to form color images in copying machines and printers using the electrophotographic technology.  The method is so adapted that toner images formed on an image bearing member are primarily transferred to an intermediate transfer member while sequentially superimposed for respective colors, and then secondarily transferred to a recording medium collectively.  In the method, a configuration using an intermediate transfer belt, formed of a belt, as an intermediate transfer member is widely adopted since the configuration includes such advantages as high freedom in arrangement and high adaptability to various media. 
As a configuration utilizing high freedom in arrangement which is an advantage of the intermediate transfer belt method, a configuration including an inner roller arranged from the inside of an intermediate transfer belt and an outer roller pressing the belt from the outside of the intermediate transfer belt has been used (Japanese Patent Application Laid-Open No.  9-222804). 
By using an outer roller in this way, the extra space inside the intermediate transfer belt can be made as small as possible, whereby the space for the main body of the apparatus can also be made small. 
However, in the case of using an intermediate transfer belt, although there is no problem when the intermediate transfer belt is driven, when the belt is stopped, the tension of the belt concentrates on the roller part.  If time passes in this state, a step made by the fact that the intermediate transfer belt is pushed along the shape of the roller, that is, a so-called curling habit, is caused.  If image formation is performed by driving the intermediate transfer belt with a curling habit, defective images such as color shift may be caused due to vibration caused when driving.