To convert an arrangement of pixels to a predetermined direction without requiring a storage position of each pixel in image data.

The image data-converting apparatus includes a line memory 104 for storing image data expressing an image comprising pixels of M lateral columns and N longitudinal rows, a detecting part for detecting data indicating the number of pixels M in the lateral direction in the image from the stored image data, and a data-converting part 106 which repeats M times by shifting a position of leading pixel data one by one, a process of repeating N times sequentially taking out pixel data of positions separated by multiples of the M from the position of the leading pixel data among the stored image data.