According to an embodiment, a sheet binding device includes a processing tray (102), a rotary paddle (1033), an adhesive application unit (101), and an abutting assistance member (1032). The processing tray stacks a sheet thereon. The rotary paddle causes the sheet to abut against a predetermined abutting alignment position on the processing tray by rotating while coming into contact with the sheet stacked on the processing tray. The adhesive application unit applies an adhesive to a predetermined area of the sheet abutted against the abutting alignment position. The abutting assistance member has such a length so as to interpose the sheet between the abutting assistance member and the rotary paddle during a time until the sheet which is conveyed toward the abutting alignment position by the rotary paddle rides on an area of the adhesive applied onto the sheet stacked on the processing tray immediately in front of the sheet.