To provide a detection method for detecting fraud, easily restoring a normal operation state while ensuring high security even if confidential data are erased due to a maintenance operation or the like.

A card reader 1 has: a removal detection switch 16 detecting that the card reader 1 is removed from a casing 4 of a user operation terminal 20; a first RAM 11 allowing stored removal detection-recognition data to be erased based on an output signal from the detection switch 16; and a second RAM 12 separate from the first RAM 11, storing authentication key data. When the card reader 1 is removed from the casing 4 of the user operation terminal 20, the removal detection-recognition data are erased, thereafter the card reader 1 is installed to the casing 4 of the user operation terminal 20, the authentication key data are changed after a predetermined authentication procedure is completed, the changed authentication key data are stored in the second RAM 12, and the removal detection-recognition data are stored in the first RAM 11.