A cassette detecting device and a medium processing device using the same are provided. The cassette detecting device comprises: a main board comprising a first input unit and detecting and outputting at least one of whether at least one cassette connected to the first input unit is mounted and its mounting position; and a plurality of cassettes storing a medium, comprising a second input unit connected to the main board, and determining the mounting position of a corresponding cassette according to a signal from the second input unit, each of the plurality of cassettes and the main board comprises a comparing unit for determining the mounting position of a cassette.