An image decoding method according to the present invention comprises the steps of: deriving quantized transform coefficients by unit of sub-blocks in a transform unit on the basis of residual information included in a bitstream; deriving transform coefficients on the basis of the quantized transform coefficients; generating a residual sample on the basis of the transform coefficients; generating a prediction sample on the basis of an inter prediction or an intra prediction; and restoring an image on the basis of the residual sample and the prediction sample. According to the present invention, a quantity of data required for a residual signal can be reduced, and an overall coding efficiency can be improved.