A medium processing apparatus is provided. The medium processing apparatus comprises an MR sensor, one or more differential amplifiers, a storage, and a controller. The MR sensor comprises a plurality of detection elements configured to detect a magnetic component of a medium. The one or more differential amplifiers receive respective output signals of two detection elements among the plurality of detection elements to subtract and amplify the received signals. The storage stores various pattern information of a magnetic component signal for the medium. The controller compares the pattern information, stored in the storage, with magnetic component signals respectively outputted from the detection elements and a magnetic component signal outputted from the one or more differential amplifiers to determine authenticity of the medium.