A method by which a video decoding device decodes a video, according to the present document, comprises the steps of: acquiring, from a bitstream, image information including video coding layer (VCL) network abstraction layer (NAL) units; performing inter prediction or intra prediction on the current block within the current picture on the basis of the image information to generate prediction samples about the current block; and restoring the current block on the basis of the prediction samples, wherein the image information includes an sps_video_parameter_set_id syntax element indicating the value of an identifier of a video parameter set, and, on the basis of that the value of the sps_video_parameter_set_id syntax element is 1, the value of the entire output layer set (OLS) number and the value of the number of layers within the OLS, specified by the video parameter set can be derived to be 1.