A medium-processing device having: a storage part, which has an aperture into which a medium is deposited, and which stores a medium deposited from the aperture; a medium-pressing part, which is capable of moving between a first end wall and a second end wall of the storage part, and which presses the medium that has been inserted from the aperture against the second end wall of the storage part; and an expansion part, which extends between the medium-pressing part and the first end wall of the storage part, and expands into a medium non-storage region between the medium-pressing part and the first end wall of the medium storage part in conjunction with the movement of the medium-pressing part from the first end to the second end of the storage part in order to press the medium (which has been deposited into a medium storage region between the medium-pressing part and the second end wall of the storage part) against the second end wall of the storage part.