A transfer belt unit satisfies the following conditions. An intermediate transfer belt has a thickness not less than 100 micrometers and not more than 200 micrometers, a tension not less than 80 N/m and not more than 180 N/m, and a tensile elastic modulus not less than 1000 megapascals and not more than 2000 megapascals. A secondary-transfer bias roller has an Asker C hardness not less than 35 degrees and not more than 50 degrees. Stretching rollers for stretching the intermediate transfer belt have an outer diameter not less than 6 millimeters.