A controlling method of a media processing apparatus, comprising: transferring and loading a medium to a media tray provided within a media processing unit which is operable to perform information processing on the medium, by using a transfer arm of a media transfer mechanism; acquiring a distance from a surface of the medium loaded on the media tray to a reference position of the transfer arm; calculating a difference between the acquired distance and a predetermined distance from a reference surface of the media tray to the reference position of the transfer arm; and determining that a double transfer of the medium to the media tray occurs when the calculated difference is largerthan a predetermined threshold value, wherein the transfer arm is provided with a media detecting unit which is operable to detect a position of the surface of the media loaded on the media tray and which includes: a detecting lever having a detecting terminal portion which is configured to come into contact with the surface of the medium and is movable from a non-detection position to a detection position when it comes into contact with the surface of the medium, and movably supported by the transfer arm; and a detector operable to detect that the detecting lever reaches the detection position; wherein the media detecting unit detects a position of the detecting [ever when the detecting lever is moved from one of the detection position and the non-detection position to the other one of the detection position and the non-detection position by the movement of the transfer arm, as the position of the surface of the medium; wherein acquiring a distance from the surface of the medium loaded on the media tray to the reference position of the transfer arm comprises: dropping the transfer arm; stoppirig the transfer arm when the detecting lever comes into contact with the surface of the media loaded on the media tray and the detecting lever is moved to the detection position; and lifting up the transfer arm until the detecting lever is moved to the non-detection position; and wherein in the acquiring, the distance from the position of the detecting lever, when the detecting lever is moved to the non-detection position, to the reference position is acquired as the distance from the surface of the medium to the reference position. transferring and loading a medium to a media tray provided within a media processing unit which is operable to perform information processing on the medium, by using a transfer arm of a media transfer mechanism; acquiring a distance from a surface of the medium loaded on the media tray to a reference position of the transfer arm; calculating a difference between the acquired distance and a predetermined distance from a reference surface of the media tray to the reference position of the transfer arm; and determining that a double transfer of the medium to the media tray occurs when the calculated difference is largerthan a predetermined threshold value, wherein the transfer arm is provided with a media detecting unit which is operable to detect a position of the surface of the media loaded on the media tray and which includes: a detecting lever having a detecting terminal portion which is configured to come into contact with the surface of the medium and is movable from a non-detection position to a detection position when it comes into contact with the surface of the medium, and movably supported by the transfer arm; and a detector operable to detect that the detecting lever reaches the detection position; a detecting lever having a detecting terminal portion which is configured to come into contact with the surface of the medium and is movable from a non-detection position to a detection position when it comes into contact with the surface of the medium, and movably supported by the transfer arm; and a detector operable to detect that the detecting lever reaches the detection position; wherein the media detecting unit detects a position of the detecting [ever when the detecting lever is moved from one of the detection position and the non-detection position to the other one of the detection position and the non-detection position by the movement of the transfer arm, as the position of the surface of the medium; wherein acquiring a distance from the surface of the medium loaded on the media tray to the reference position of the transfer arm comprises: dropping the transfer arm; stoppirig the transfer arm when the detecting lever comes into contact with the surface of the media loaded on the media tray and the detecting lever is moved to the detection position; and lifting up the transfer arm until the detecting lever is moved to the non-detection position; and dropping the transfer arm; stoppirig the transfer arm when the detecting lever comes into contact with the surface of the media loaded on the media tray and the detecting lever is moved to the detection position; and lifting up the transfer arm until the detecting lever is moved to the non-detection position; and wherein in the acquiring, the distance from the position of the detecting lever, when the detecting lever is moved to the non-detection position, to the reference position is acquired as the distance from the surface of the medium to the reference position.