To solve a problem in which, since an introduction pass for guiding a sheet guides the sheet to the downstream side of a stapler, the introduction pass and the stapler are arranged in the vertical direction and the size of a sheet processing device is increased.

In a sheet processing device 11, a downstream end 44a of an introduction pass 44 is connected to a folding and conveying passage 81, and an inlet deflector 82 for guiding a sheet to a connection part T1 is arranged therein. The inlet deflector 82 permits the advance of the sheet to the stapler 31 from the introduction pass 44, prevents the advance of the sheet to the introduction pass 44 from the stapler 31, and guides the sheet to the conveying passage 81.

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