A data transmission system or an electronic camera in which data is transmitted from a first block to a second block is disclosed. A requesting means notifies from the second block, a request of receiving data from the first block at least in predetermined units thereto. A reserving means notifies, from the first block, the readiness of outputting data to the second block, in predetermined units thereto. A data outputting means outputs, from the first block, data to the second block on the basis of the notifications from the requesting means and the reserving means. Thus, it is possible to efficient and effective data transmission between two data processing blocks.