An image decoding method performed by a decoding device according to the present document comprises a step of receiving a bitstream containing residual information, and a step of deriving a quantized transform coefficient for a current block on the basis of the residual information contained in the bitstream, wherein the residual information includes a context syntax element coded on the basis of the context, and if the size of a transformation block is changed as much as a sub-block region having a predetermined size and the current block is the last sub-block in the sub-block region and the last quantized transform coefficient is in the current block, then the step of deriving a quantized transform coefficient includes context-based decoding of the context syntax element on the basis of a predetermined maximum value for the context syntax element, wherein the maximum value is determined on the basis of the size of the sub-block region.