A method by which a video decoding device decodes a video, according to the present document, can comprise the steps of: parsing, from a bitstream, number information related to the number of slices of which the height within a tile of a current picture is explicitly signaled; parsing, from the bitstream, on the basis of the number information, height information related to the height of the slices of which the height is explicitly signaled; deriving the number of slices in the tile on the basis of the number information and the height information; generating prediction samples by predicting the current block of the current picture on the basis of the slices within the tile; generating reconstructed samples on the basis of the prediction samples; and generating a reconstructed picture for the current picture on the basis of the reconstructed samples.