an image - transmitting - side device comprises : a one - phase to two - phase converter circuit for separating parallel image data , which are to be transmitted , into even and odd data ; a first parallel - serial converting circuit ; a second parallel - serial converting circuit ; means for allowing a user to select , as the resolution mode for the image data to be transmitted , one of a first resolution mode and a second resolution mode that is higher in resolution than the first resolution mode ; and switch means for applying the parallel image data , which are to be transmitted , to the first parallel - serial converting circuit when the first resolution mode is selected , and for applying the parallel image data , which are to be transmitted , to the one - phase to two - phase converter circuit when the second resolution mode is selected .