An image data converting device of at least one embodiment of the present invention for converting, by linear interpolation, input image data having a predetermined resolution into delta arrangement image data having a resolution that is smaller than the predetermined resolution, the image data converting device includes: an even line pixel value converting section for converting a value of each pixel in each even line in the input image data into a value of each pixel in each even line in the delta arrangement image data, by using a predetermined initial value for even lines; and an odd line pixel value converting section for converting a value of each pixel in each odd line in the input image data into a value of each pixel in each odd line in the delta arrangement image data, by using a predetermined initial value for odd lines, the predetermined initial value for odd lines being (1±α)/2(0≦α≦0.5) of the initial value for the even lines. This allows conversion into an image that looks natural by controlling a jaggy and/or colored appearance of a contour section of the image, when image data having a high resolution is converted into the delta arrangement image data having a low resolution.