A method by which a decoding device performs image decoding, according to the present document, comprises the steps of: deriving a reference block on the basis of a motion shift; determining whether a subblock temporal motion vector predictor (sbTMVP) candidate is available, on the basis of whether the reference block is a current picture referencing (CPR) block; deriving motion information about a current block on the basis of the sbTMVP candidate when the sbTMVP candidate is available; generating prediction samples on the basis of the motion information; and generating a restored picture on the basis of the prediction samples, wherein the sbTMVP candidate is determined to be unavailable with respect to the reference block which is the CPR block.