A medium processing device including a holding section that includes an opening forming a medium insertion port, and that holds a medium inserted through the insertion port, a medium press section that is capable of moving between a first end side wall and a second end side wall of the holding section, and that presses the medium inserted through the insertion port against the second end side wall of the holding section, and an opening-out section that spans between the medium press section and the first end side wall of the holding section, and that opens out in a medium non-holding area between the medium press section and the first end side wall of the holding section accompanying movement of the medium press section from the first end side to the second end side of the holding section in order to press the medium inserted into a medium holding area between the medium press section and the second end side wall of the holding section against the second end side wall of the holding section.