A medium processing device includes: a transaction unit including a transaction port and configured to perform at least one of a process of taking in a medium or a process of taking out the medium through the transaction port; a cassette mounting unit including a mounting port wherein a medium cassette is mounted to the mounting port; a cassette processing unit configured to perform at least one of a process of carrying in the medium from the medium cassette mounted to the cassette mounting unit or a process of carrying out the medium to the medium cassette; and a closing member configured to selectively close the transaction port or the mounting port.