1. A method of entropy-decoding image data, the method comprising: entropy-decoding a syntax element of a first block at a first position within an layer by using first probability information; obtaining second probability information by updating the first probability information; and entropy-decoding a syntax element of a second block at a second position within the layer and a syntax element of a third block at a third position within the layer by using the second probability information, wherein the first block is decoded prior to the second block and the third block, and wherein the second position within the layer of the second block and the third position within the layer of the third block are spatially separated positions from each other within the layer, wherein the layer is one of layers included in an image, and wherein the first, second and third blocks are coding tree units (CTU) having a predetermined size.