1. An image data transfer method for sending image data recorded in a recording medium by a digital camera communicable with data storage means to the data storage means that has a function of remote communication, the image data transfer method comprising: adding ID information for identifying an owner of the digital camera to the image data; sending the image data added with the ID information to a user directory that corresponds to the ID information in the data storage means; receiving a confirmation representing that the image data added with the ID information are stored in the data storage means; marking the image data in the recording medium to indicate that the image data are stored in the data storage means, wherein marking the image data comprises turning archive properties of the image data OFF; and determining whether an archive property of the image data is set to OFF, and if affirmative, performing a size-reduction processing within the digital camera to generate reduced-sized image data, storing the reduced-sized image data in the recording medium of the digital camera, and deleting the image data corresponding to the reduced-sized image data from the recording medium of the digital camera.