The present invention includes an image information decoding method which comprises: a step of receiving a bitstream that includes a network abstraction layer (NAL) unit including information related to an encoded image; and a step of parsing an NAL unit header of the NAL unit. The NAL unit header includes layer information including reserved_one_5bits for identifying an extended layer in an extended bitstream and temporal_id for identifying a temporal layer of a bitstream. The reserved_one_5bits of the layer information is received prior to the temporal_id of the layer information. Thus, a method for describing scalability information in a hierarchical bitstream is provided.