A one-side or both-side transmission mode is properly automatically selected in accordance with an original, thereby enabling desired image communication of the user to be properly executed. A CPU discriminates whether effective image data is included on both sides of an original image read by a reading circuit or not. If at least one sheet of original in which the effective image data is included on both sides exists, a both-side transmission of the image data read from both sides of the original is executed, and in the other cases a one-side transmission of the image data read from the front side of the original is executed. When a compulsory front side reading mode is selected by the operation of an operation unit, even if at least one sheet of original in which the effective image data is included on both sides exists, only the front side of the original is read and the read image data is transmitted in the one-side mode.