1. An apparatus of decoding video, the apparatus comprising: a parser which obtains, from a bitstream, a split transformation flag for a current depth, and when the split transformation flag indicates non-split for the current depth, determines a transformation depth is equal to the current depth; a context modeler which determines a context increasement parameter for determining a context index based on whether the transformation depth is equal to a predetermined value without using a size of a transformation unit, and obtains a context model for arithmetic decoding a transformation unit significant coefficient flag using the context index obtained by adding the context increasement parameter and a context offset; and an arithmetic decoder which arithmetic decodes the transformation unit significant coefficient flag based on the context model, and determines whether at least one non-zero transformation coefficient exists in the transformation unit of the transformation depth, based on the transformation unit significant coefficient flag, wherein the transformation depth indicates a number of times a height and a width of a coding unit are split to obtain the transformation unit, wherein the predetermined value is zero, and wherein, when the split transformation flag indicates split for the current depth, a transformation unit of the current depth is split into one or more transformation units of a next depth, and a split transformation flag for the next depth is obtained from the bitstream.