A method of controlling an image data output of a portable device, a portable device having a signal detection unit and image processing unit are provided. The signal detection unit generates a photographing-disabled mode signal in response to a restricted area signal. The image processing unit has a photographing-disabled mode decision unit, an authentication decision unit and a control unit. The photographing-disabled mode decision unit determines whether the portable device is in the photographing-disabled mode or not based on the photographing-disabled mode signal. The authentication decision unit determines whether the portable device is authenticated or not when the portable device is in the photographing-disabled mode. The control unit outputs first photographed image data when the portable device is authenticated, and prohibits an output of the first photographed image data when the portable device is not authenticated.