To obtain a closed-loop resin belt for a fixing device and a transfer- intermediate transfer device conducting an image fixing operation and a transfer-intermediate transfer operation, with excellent productivity and at a low cost, to prepare a resistance-control endless resin belt excellent in the uniformity of the film thickness distribution of the whole belt, irrespective of the kind of a sheetlike film being used, and to obtain an image forming apparatus capable of outputting a highly minute image, by combining a fixing unit and a transfer-intermediate transfer unit using the resin belt.

In regard to a method of preparing the closed-loop resin belt by winding a thermoplastic sheetlike film on a columnar member and by welding the film by heating, a thin film member is provided at the end of the film on the occasion of winding the film and the amount of movement of a resin in the end is thereby controlled, so as to always control the film thickness distribution of the whole belt with high precision, irrespective of the kind of the film being used, in the present manufacturing method of the resin belt provided herein.