An image coding apparatus to reduce data involved in coding a picture having a bit-depth higher than a bit-depth supported by an encoder, includes a down-converter which generates a base layer picture by down-converting a target picture to reduce the bit-depth of the target picture by a first value, a first encoder codes the base layer picture to generate a reconstructed picture by decoding the coded base layer picture, a subtractor performs shift amount adjustment on the reconstructed picture to increase the bit-depth of the reconstructed picture by a second value, and generates, based on the reconstructed picture on which the adjustment has been performed and the target picture, an enhancement layer picture for enhancing the quality of the base layer picture, a second encoder codes the enhancement layer picture; and an output unit outputs the coded base layer picture and the coded enhancement layer picture.