An image decoding method performed by a decoding device according to the present document is characterized by comprising the steps of: acquiring a flag indicating whether a picture header (PH) network abstraction layer (NAL) unit including a PH for the current picture is present; acquiring a video coding layer (VCL) NAL unit including a slice header and slice data for the current slice of the current picture; acquiring the PH on the basis of the flag; and decoding the current picture on the basis of the PH, the slice header, and the slice data.