1. An image decoding apparatus comprising: an entropy decoding unit which decodes image data including information on a quantization parameter from a bitstream; 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 picture are inverse-quantized using a second quantization parameter obtained based on a first quantization parameter, with all other macroblocks of the picture being inverse-quantized using the first quantization parameter decoded from the bitstream; and an inverse-transform unit which inverse-transforms the inverse-quantized macroblocks.