Making effective use of an image encoder and an image decoder for processing a color image of a general-purpose standard bit depth, an image transmission device capable of transmitting/receiving a monochrome image of a higher bit depth is configured. An image transmission device includes an image encoder to encode a high bit-depth monochrome image and output encoded data and an image decoder to generate, by decoding the encoded data received via a transmission path, a high bit-depth monochrome image. The image encoder decomposes the input high bit-depth image data into plural bit planes corresponding to color image data of a standard bit depth and encodes the standard bit-depth color image data. The image decoder decodes the color image data of the standard bit depth and synthesizes, from the decoded standard bit-depth color image data, a high bit-depth monochrome image.