1. An image data synchronizing apparatus comprising: an extraction device for extracting one horizontal line of image data in a plurality of image areas, which are obtained when one frame of an image has been divided in the horizontal direction, on a per-image-area basis from applied image data representing one frame of the image; and a line memory for storing temporarily the image data that has been extracted by said extraction device, said line memory having a capacity for an amount of data approximately equal to that of the one horizontal line of image data in each image area.