An image data recorder coding a still image in accordance with a moving image coding system, and recording a still-image-converted moving image acquired by coding onto a recording medium includes: a reading unit reading out an aspect ratio set for the recording medium; a determining unit acquiring an aspect ratio of the still image, and determining whether the acquired aspect ratio is the same as the aspect ratio for the recording medium; a coding unit adding an interpolation area on the top and bottom, or left and right of the still image in accordance with the result of the determination to generate an interpolation-area-added still image whose aspect ratio matches the aspect ratio for the recording medium, and coding the interpolation-area-added still image in accordance with a moving image coding system to generate the still-image-converted moving image; and a recording unit recording the still-image-converted moving image onto the recording medium.