A method by which a decoding device decodes an image, according to the present document, comprises the steps of: receiving a bitstream including residual information about a current block; decoding syntax elements for the current block included in the residual information; deriving conversion coefficients for the current block on the basis of the decoded syntax elements; deriving residual samples for the current block on the basis of the conversion coefficients; and generating a restoration picture on the basis of the residual samples.