A document scanning device reads an image on a document while scanning the image, and generates image data based on the image. The document scanning device selects and sets a scanning speed in accordance with a transfer mode used when the document scanning device transfers the image data to another apparatus. The document scanning device controls the amount of generated image data by using the set scanning speed to successively read the image, whereby optimal transfer processing in accordance with the transfer mode can be performed. By switching a transfer path in accordance with the type of the read data, i.e., binary data per pixel or multilevel data per pixel, efficient data transfer can be performed.