An image forming apparatus comprising: a drum-shaped image carrier; a transfer and separation device which comprises: a conveyor belt for conveying a transfer material; a transfer roller for supporting said conveyor belt; and a nip regulating roller that is disposed downstream of said transfer roller in a conveyance direction of said transfer material, wherein said transfer and separation device, nips said transfer material with said image carrier and said conveyor belt by pressing said conveyor belt and said transfer material against said image carrier with said transfer roller, and regulates a transfer region (AP to BP) in which said transfer material is brought in close contact with said image carrier by adjusting said conveyor belt to said image carrier; and characterized in a transfer region changing section adapted to change a downstream end (point BP) of the transfer region in a conveyance direction of the transfer material by changing a position of the nip regulating roller; and a control section for controlling the transfer region changing section in accordance with thickness of the transfer material; said control section is so configured that said nip regulating roller is at a first position close to the surface of said image carrier in case regular or thin paper is used, and said nip regulating roller is at a second position further away from said surface of said image carrier compared to said first position in case thick paper is used, and said conveyor belt at the downstream end of said transfer region (point BP) moves parallel to the tangential line drawn at the downstream end (point BP) on said surface of said image carrier, in case regular or thin paper is used and said nip regulating roller is at said first position, and said downstream end of said transfer region (point BP) is moved further to the upstream side in case thick paper is used, a drum-shaped image carrier; a transfer and separation device which comprises: a conveyor belt for conveying a transfer material; a transfer roller for supporting said conveyor belt; and a nip regulating roller that is disposed downstream of said transfer roller in a conveyance direction of said transfer material, wherein said transfer and separation device, nips said transfer material with said image carrier and said conveyor belt by pressing said conveyor belt and said transfer material against said image carrier with said transfer roller, and regulates a transfer region (AP to BP) in which said transfer material is brought in close contact with said image carrier by adjusting said conveyor belt to said image carrier; and characterized in a transfer region changing section adapted to change a downstream end (point BP) of the transfer region in a conveyance direction of the transfer material by changing a position of the nip regulating roller; and a control section for controlling the transfer region changing section in accordance with thickness of the transfer material; said control section is so configured that said nip regulating roller is at a first position close to the surface of said image carrier in case regular or thin paper is used, and said nip regulating roller is at a second position further away from said surface of said image carrier compared to said first position in case thick paper is used, and said conveyor belt at the downstream end of said transfer region (point BP) moves parallel to the tangential line drawn at the downstream end (point BP) on said surface of said image carrier, in case regular or thin paper is used and said nip regulating roller is at said first position, and said downstream end of said transfer region (point BP) is moved further to the upstream side in case thick paper is used, a conveyor belt for conveying a transfer material; a transfer roller for supporting said conveyor belt; and a nip regulating roller that is disposed downstream of said transfer roller in a conveyance direction of said transfer material, wherein said transfer and separation device, nips said transfer material with said image carrier and said conveyor belt by pressing said conveyor belt and said transfer material against said image carrier with said transfer roller, and regulates a transfer region (AP to BP) in which said transfer material is brought in close contact with said image carrier by adjusting said conveyor belt to said image carrier; and characterized in a transfer region changing section adapted to change a downstream end (point BP) of the transfer region in a conveyance direction of the transfer material by changing a position of the nip regulating roller; and a control section for controlling the transfer region changing section in accordance with thickness of the transfer material; said control section is so configured that said nip regulating roller is at a first position close to the surface of said image carrier in case regular or thin paper is used, and said nip regulating roller is at a second position further away from said surface of said image carrier compared to said first position in case thick paper is used, and said conveyor belt at the downstream end of said transfer region (point BP) moves parallel to the tangential line drawn at the downstream end (point BP) on said surface of said image carrier, in case regular or thin paper is used and said nip regulating roller is at said first position, and said downstream end of said transfer region (point BP) is moved further to the upstream side in case thick paper is used,