Belt conveyance device and image forming apparatus

An image forming apparatus comprising:

• an image forming portion configured to form an image;
• a belt unit dismountably mounted on a main assembly of the image forming apparatus,
• wherein the belt unit includes:
• an endless belt to which a toner image formed in the image forming portion is transferred;
• a tension mechanism capable of applying a tension to the belt and including a tension roller configured to apply the tension to the belt by stretching the belt and movably provided between a first position where the tension is applied to the belt and a second position where the tension of the belt is released or the tension of the belt is loosened more than when the tension roller is positioned in the first position, and an urging member configured to urge the tension roller;
• an engaged portion provided on the tension mechanism and configured to be engaged with a tension releasing member configured to restrict movement of the tension roller from the second position to the first position,
• wherein the tension releasing member permits the movement of the tension roller from the second position to the first position by engagement with the tension mechanism being released;
• and
• a contacted portion provided on the main assembly of the image forming apparatus and configured to prevent the belt unit from being mounted by the tenson releasing member being contacted when the belt unit is to be mounted in a state in which the tension releasing member is engaged with the tension mechanism.