1. An image decoding apparatus comprising: an entropy decoding unit which decodes image data including information on a quantization parameter for each macroblock in a bitstream; and an inverse-quantizing unit which determines respective quantization parameters based on a location of each macroblock and inverse-quantizes each macroblock using the respectively determined quantization parameter, wherein macroblocks located on a boundary of a region are inverse-quantized using a second quantization parameter obtained based on a first quantization parameter, with all other macroblocks of the region being inverse-quantized using the first quantization parameter decoded from the bitstream for macroblocks of the region.