An image forming apparatus includes an image bearing member for bearing a toner image; a movable intermediary transfer belt extended around a plurality of supporting shafts, the intermediary transfer belt includes a core member and elastic layers sandwiching the core member, wherein a toner image on the image bearing member is transferred onto the intermediary transfer belt, and the toner image on the intermediary transfer belt is transferred onto a transfer material; wherein a distance r (mm) between a center of such one of the supporting shafts as has a minimum diameter and a center of the core member, a distance L.sub.1 (mm) between the center of the core member and a surface of the intermediary transfer belt where toner image transfer occurs, a distance L.sub.2 (mm) between a surface Of the core member and the surface of the intermediary transfer belt where the toner image transfer occurs, and a size t (mm) of the core member measured in a direction perpendicular to a movement direction of the intermediary transfer belt, satisfy:L.sub.1 .ltoreq.0.1r, and0.5t.ltoreq.L.sub.2.A method for manufacturing the intermediary transfer belt is also a part of applicants improvement in the art to which the invention pertains.