A video decoding method, by a decoding apparatus, the method comprising: obtaining video information including picture reconstruction information and offset information from a bitstream; deriving a prediction mode of a current block in a current picture as an inter prediction mode based on the picture reconstruction information; deriving a reference block of the current block based on the picture reconstruction information; deriving a predicted block by performing inter prediction on the current block based on the reference block; generating a reconstructed picture of the current picture based on the predicted block; and applying an in-loop filter to the reconstructed picture, wherein the step of applying the in-loop filter includes the steps of: applying a deblocking filtering process to the reconstructed picture; and applying a sample adaptive offset process to the reconstructed picture based on the offset information after completing the deblocking filtering process, wherein the offset information includes a sample adaptive offset process flag indicating that the sample adaptive offset process is enabled, wherein the offset information includes offset type information for specifying an offset type for a sample, wherein the offset type information indicates a band offset for the sample, wherein the offset information includes information on n offset values, characterized in that the offset information includes start band information explicitly indicating one of 32 bands as a start band of n bands to determine a band group comprising the n bands among the 32 bands, the start band information being different from the offset type information, the band group is determined further based on end band information variably indicating an end band of the n bands, wherein the band group is variably located in among the 32 bands based on the start band information, wherein an offset value corresponds to a band among the n bands for the sample, n is a positive integer. obtaining video information including picture reconstruction information and offset information from a bitstream; deriving a prediction mode of a current block in a current picture as an inter prediction mode based on the picture reconstruction information; deriving a reference block of the current block based on the picture reconstruction information; deriving a predicted block by performing inter prediction on the current block based on the reference block; generating a reconstructed picture of the current picture based on the predicted block; and applying an in-loop filter to the reconstructed picture, wherein the step of applying the in-loop filter includes the steps of: applying a deblocking filtering process to the reconstructed picture; and applying a sample adaptive offset process to the reconstructed picture based on the offset information after completing the deblocking filtering process, applying a deblocking filtering process to the reconstructed picture; and applying a sample adaptive offset process to the reconstructed picture based on the offset information after completing the deblocking filtering process, wherein the offset information includes a sample adaptive offset process flag indicating that the sample adaptive offset process is enabled, wherein the offset information includes offset type information for specifying an offset type for a sample, wherein the offset type information indicates a band offset for the sample, wherein the offset information includes information on n offset values, characterized in that the offset information includes start band information explicitly indicating one of 32 bands as a start band of n bands to determine a band group comprising the n bands among the 32 bands, the start band information being different from the offset type information, the band group is determined further based on end band information variably indicating an end band of the n bands, wherein the band group is variably located in among the 32 bands based on the start band information, wherein an offset value corresponds to a band among the n bands for the sample, n is a positive integer.