A card medium processing device is controlled by means of the control method including: a first judgment step for judging whether or not a card medium having been inserted is detected by a first detection means for detecting the card medium having been inserted; a second judgment step for judging whether or not the card medium having been inserted is detected by a second detection means for detecting the card medium having been inserted, the second detection means being placed at a position behind the first detection means further away toward a rear side in an inserting direction for the card medium; and a processing step for carrying out a predetermined process, supposing that the first detection means has an error, under a situation where the first judgment step does not detect the card medium having been inserted while the second judgment step detects the card medium having been inserted.