A sheet post-processing device includes a conveying member, a processing tray, a processing section, a reference plate, an alignment member, a holder drive unit, and a control unit. The alignment member includes a paddle holder, and an alignment paddle having a paddle main body supported in a rotatable manner at the paddle holder, and a paddle part protruding from the paddle main body. The holder drive unit moves the alignment paddle in a reciprocating manner, between a reference position to separate the paddle part above a top sheet and an acting position to allow the paddle part to contact with the top sheet. The control unit sets a second acting position when the paddle part contacts with the sheet at n1-th time, to be higher than a first acting position when the paddle part contacts with the sheet at m1-th time, where 1≤m1