Present invention provides a medium processing device and a medium transaction device that may perform conveyance processing due to input and output of mediums with simple configuration. In a banknote deposit/withdrawal device of ATM, conveyance path inside a conveyance section is formed in straight line shape running mainly along front-rear direction. During deposit counting processing of deposit processing, the banknote deposit/withdrawal device retains deposit reject banknotes in a conveyance path retention section within a second conveyance section, and decides and stores conveyance destination for each banknote based on verification results obtained from a verification section, and during deposit storage processing of deposit processing, the banknote deposit/withdrawal device conveys banknotes fed out from a temporary holding section to the decided conveyance destinations to be stored. During withdrawal processing, the banknote deposit/withdrawal device decides banknote conveyance destinations based on detection results of a run sensor of the verification section.