Image forming apparatus including a movable belt

An image forming apparatus for forming an image on a recording material by forming a toner image on a photosensitive member, comprising:

• a main assembly;
• a cartridge including said photosensitive member;
• an endless belt configured to transfer the toner image formed on said photosensitive member;
• an endless belt supporting member configured to support said endless belt so as to be movable in a contacting position for contacting said photosensitive member and a non-contacting position retracted from the contacting position away from said photosensitive member;
• and
• a cartridge supporting member dismountably supporting said cartridge and movable between an image forming position in which said photosensitive member contacts said endless belt taking the contacting position and a non-image-forming position in which said cartridge is movable from a inside of said main assembly to a outside of said main assembly,
• wherein by movement of said endless belt from the contacting position to the non-contacting position, a space configured to accommodate at least a part of said cartridge supporting member taking the non-image-forming position and/or said cartridge supported by said cartridge supporting member taking the non-image-forming position is formed,
• wherein said endless belt supporting member includes a plurality of stretching members supporting said endless belt and configured to move to release said endless belt from a stretched state to allow said endless belt to move from the contacting position to the non-contacting position by a weight of said endless belt.