A post-processing apparatus that rotates a rotation paddle in which a fin having flexibility is cantilevered by a holder to apply a conveying force along a rotation direction to a sheet to align the sheet, wherein the holder includes a recess in which a peripheral surface part from a support position of the fin to an upstream side in the rotation direction is recessed from a circumscribed circle of the holder, and when the fin elastically turns while being pressed against the sheet with rotation of the rotation paddle, turning of the fin is allowed until the fin comes into contact with a surface of the recess.

A sheet post-processing device includes a conveying member, a processing tray, a processing section, a reference plate, an alignment member, and a frame. The processing tray stacks sheets carried in by the conveying member. The alignment member moves the sheet carried in onto the processing tray in an alignment direction so as to aid alignment of the sheets. The alignment member includes a roller holder, an alignment roller, and a guide member. The guide member has a sheet guide part protruding from a swinging end of the roller holder to the downstream side in the alignment direction than the alignment roller. When the number of the sheets placed on the processing tray varies, an angle of the guide member with respect to the roller holder varies so that a distance between a top sheet placed on the processing tray and the sheet guide part becomes substantially constant.