Medium processing device, information processing device, and medium processing system

In order to enable detection of a new counterfeit medium, a control unit (10) of a paper currency sorting device (1) stores, in a processed serial number list (L1), the serial numbers of paper currency that has undergone a sorting process in a previous paper currency sorting process, and in the paper currency sorting process the control unit compares the serial numbers of the paper currency being sorted to the serial numbers in the processed serial number list (L1). When a serial number matches a serial number in the processed serial number list (L1), the control unit (10) transports the paper currency in question to a first reject unit (17), stops the paper currency sorting process, and displays a warning screen (D1) on a display/operation unit (11). Thus, the paper currency sorting device (1) is able to detect paper currency that has not yet been registered in a counterfeit list, is able to discriminate the paper currency being sorted from legitimate paper currency, and, by performing a preparatory process for discrimination of processed paper currency for which there is a serial number match, enables an employee or the like to discriminate counterfeit currency easily.