A detection method for detecting fraud with respect to a card reader. The card reader includes a removal detection switch for detecting a removal from a housing of a user operation terminal, a first RAM that can erase removal detection recognition data being stored therein, according to an output signal from the detection switch, a second RAM being independent of the first RAM and storing authentication key data. The method includes erasing the removal detection recognition data is erased when the card reader is removed from the housing of the user operation terminal. Afterwards, the authentication key data is changed after completion of predetermined authentication procedures when the card reader is mounted into the housing of the user operation terminal. Then, the changed authentication key data is stored in the second RAM, while the removal detection recognition data is stored in the first RAM.