An image data recording/playback device, including a decoder that decodes image data encoded in a first format; and a transcoder capable of encoding image data, supplied from the decoder and encoded in the first format, into a second format whose compression rate is higher than the compression rate of the first format and outputting the image data encoded into a second format to the decoder and, of decoding image data supplied from the decoder and encoded in the second format, outputting it to the decoder, and alternately carrying out the encoding processing and the decoding processing, wherein: the decoder inputs/outputs image data encoded in the first and second format; and the decoder simultaneously inputs or outputs image data encoded in the first format when inputting or outputting image data encoded in the second format.