To provide a sheet treatment device capable of improving the loading state of the sheets P on a stack tray 28a without generating loading failure such as rear end leaning and popping out even in the specific sheet such as thin paper and board.

Rear end detection sensors 301, 302 are provided and a rear end position of the sheets P delivered from a pair of bundle delivery rollers 127 to the stack tray 128a is discriminated to the three states of a proper stage, a too little stage and a too big stage. In the case of the proper stage, the rotation speed of the bundle delivery roller 127 is maintained as it is, but in the case of the too little stage, the rotation speed is raised by one stage to increase the popping amount of the sheet P and in the case of the too big stage, the rotation speed is reduced by one stage to reduce the popping amount. The operation is repeated to converge the popping amount to a proper range and the sheet P is reached/dropped to a proper range on the stack tray 128a.