An image coding apparatus (100 100A) which codes a target picture (D31A) having a bit-depth higher than a predetermined bit-depth, the bit-depth indicating an information capacity per pixel, the image coding apparatus (100 100A) comprising: a down-converter which generates a base layer picture (D32A) by performing down-conversion on the target picture (D31A) to reduce the bit-depth of the target picture by a first value so that the bit-depth of the target picture (D31A) equals the predetermined bit-depth; a first encoder (111 111A) which codes the base layer picture (D32A), and generates a reconstructed picture (D33A) by decoding the coded base layer picture (D32A); a subtractor, the subtractor comprising an up-converter which performs shift amount adjustment on the reconstructed picture (D33A) to increase a bit-depth of the reconstructed picture by a second value, b, the subtractor generating an enhancement layer picture (D34A) which is a picture for enhancing a quality of the base layer picture (D32A), by subtracting the reconstructed picture (D33A) on which the shift amount adjustment has been performed from the target picture (D31A), the enhancement layer picture being clipped at the bit depth of the base layer picture (D32A); a second encoder (112 112A) which codes the enhancement layer picture (D34A) and the second value, b; and, an output unit (101 115) which outputs the coded base layer picture and the coded enhancement layer picture and the coded second value, b, the second value, b, is arranged to indicate a variable with value greater than or equal to 0 and less than the first value, and, wherein the subtractor (i) subtracts, from each of pixel values of pixels included in a picture having a bit-depth obtained by reducing the bit-depth of the target picture (D31A) by a third value, a pixel value of a corresponding one of pixels included in the reconstructed picture on which the shift amount adjustment has been performed, and (ii) generates the enhancement layer picture (D34A) using low N bit out of bit information indicating a result of the subtraction, N indicating the bit-depth of the base layer picture (D32A) and the third value being a difference between the first value and the second value, b, characterized in that, the subtractor includes a shift amount determining unit configured to determine the second value, b, based on the luminance variance of the base layer picture (D32A), and, the output unit (101 115) is further configured to output a value corresponding to the second value, b, determined by the shift amount determining unit, wherein the shift amount determining unit is configured to determine the second value, b, such that the second value, b, increases in regions of low luminance variance, in which low frequency components among spatial-frequency components of the base layer picture are visually more dominant. a down-converter which generates a base layer picture (D32A) by performing down-conversion on the target picture (D31A) to reduce the bit-depth of the target picture by a first value so that the bit-depth of the target picture (D31A) equals the predetermined bit-depth; a first encoder (111 111A) which codes the base layer picture (D32A), and generates a reconstructed picture (D33A) by decoding the coded base layer picture (D32A); a subtractor, the subtractor comprising an up-converter which performs shift amount adjustment on the reconstructed picture (D33A) to increase a bit-depth of the reconstructed picture by a second value, b, the subtractor generating an enhancement layer picture (D34A) which is a picture for enhancing a quality of the base layer picture (D32A), by subtracting the reconstructed picture (D33A) on which the shift amount adjustment has been performed from the target picture (D31A), the enhancement layer picture being clipped at the bit depth of the base layer picture (D32A); a second encoder (112 112A) which codes the enhancement layer picture (D34A) and the second value, b; and, an output unit (101 115) which outputs the coded base layer picture and the coded enhancement layer picture and the coded second value, b, the second value, b, is arranged to indicate a variable with value greater than or equal to 0 and less than the first value, and, wherein the subtractor (i) subtracts, from each of pixel values of pixels included in a picture having a bit-depth obtained by reducing the bit-depth of the target picture (D31A) by a third value, a pixel value of a corresponding one of pixels included in the reconstructed picture on which the shift amount adjustment has been performed, and (ii) generates the enhancement layer picture (D34A) using low N bit out of bit information indicating a result of the subtraction, N indicating the bit-depth of the base layer picture (D32A) and the third value being a difference between the first value and the second value, b, characterized in that, the subtractor includes a shift amount determining unit configured to determine the second value, b, based on the luminance variance of the base layer picture (D32A), and, the output unit (101 115) is further configured to output a value corresponding to the second value, b, determined by the shift amount determining unit, wherein the shift amount determining unit is configured to determine the second value, b, such that the second value, b, increases in regions of low luminance variance, in which low frequency components among spatial-frequency components of the base layer picture are visually more dominant.