A data transmission method able to maintain the quality of a transmitted image substantially same as that of one's own image and an apparatus using the method and a data transmission system, transmitting image data accommodated in a frame of a preset frame during communication between terminals, comprising capturing an image including a target image to be transmitted, making adjustments so that the target image to be transmitted from the obtained image substantially fully fills the image frame, and compressing and transmitting the adjusted image data in the image frame.