To provide a sheet processor that can be easily and accurately mounted in a sheet ejection space in an image forming device, and a method for mounting the sheet processor.

In the image forming device in which a scanner 3 which reads an image of a document is disposed on an image forming part 3 which forms an image on a sheet and the sheet ejection space 4 which ejects the sheet is positioned between the image forming part 1 and scanner 3, the sheet processor which can be mounted in the sheet ejection space 4 is provided. The sheet processor can be divided into an option sheet ejection unit 160 connected to a sheet ejection path provided in an upper face of the image forming part 1, and a finisher main body 200 which processes the sheet sent from the option sheet ejection unit 160. The option sheet ejection unit 160 is mounted from the exterior of a side of a device main body. The finisher main body 200 is mounted from the front of the device main body.