An image decoding method performed by a decoding device according to the present document comprises the steps of: deriving a value of a variable on the basis of whether a current picture is a first picture of a current access unit (AU) that is a coded video sequence start access unit (CVSS) AU instead of AU 0, wherein the variable indicates whether all picture storage buffers within a decoded picture buffer (DPB) are emptied without being output; updating the DPB on the basis of the variable; and decoding the current picture on the basis of the updated DPB.