An image decoding method performed by a decoding device according to the present document is characterized by comprising the steps of: acquiring image information through a bitstream; and decoding the current picture on the basis of the image information. The step for acquiring the image information includes the steps of: acquiring a flag indicating whether a picture header (PH) network abstraction layer (NAL) unit for the current picture is present; and acquiring the PH for the current picture on the basis of the flag, wherein the NAL unit including the PH is derived on the basis of the flag.