A scanner apparatus 1 stores image data on raster basis in a buffer 12 and transfers it to a host apparatus. The scanner apparatus 1 includes means 16 for temporarily stopping read of image data when an empty capacity of the buffer 12 has become smaller than a predetermined capacity, means 17 for writing, in response to the temporary stop, predetermined mark data on a storage line next to the last raster image data which has been read and stored in the buffer 12, and means 16 for resuming the image data read by overlapping the last raster image data which has been read, when the empty capacity of the buffer 12 has become greater than the predetermined capacity.