PURPOSE: To attain image data transfer synchronized among devices with different data transfer rates.

CONSTITUTION: This device is provided with a control means for selecting a scanning rate and a line sample rate, and an image means functionally connected with the pertinent control means, for scanning a document at the selected scanning rate, and generating valid image data with a set of N (numbers more than 2) scanning lines. Moreover, this device is provided with a buffer means functionally connected with the pertinent control means, for storing the scanning lines in a smaller number than the N according to the selected line sample rate, and a means functionally connected with the pertinent control means, for outputting the storage scanning lines of the image data to a host terminal. Then, video data are transferred from an image input terminal to the host terminal having a transfer rate slower than the data transfer rate of the pertinent image input terminal.