1. A video decoding apparatus comprising: a receiver which receives a bitstream including a transformation unit split flag indicating whether a transformation unit is split, the transformation unit being included in a coding unit; a context modeler which determines a context model based on a size of the transformation unit; an entropy decoder which obtains the transformation unit split flag by entropy decoding the bitstream based on the determined context model; and an image decoder which, when the transformation unit split flag indicates a split of a transformation unit of a current transformation depth, splits the transformation unit of the current transformation depth into a plurality of transformation units of lower transformation depth, and when the transformation unit split flag indicates a non-split of a transformation unit of a current transformation depth, performs inverse-transformation on the transformation unit of the current transformation depth.