An image decoding method performed by a decoding apparatus, the method comprising: receiving information on a parallel merge level which represents a size of a parallel merging unit region and merge index information; obtaining merge flag information which represents whether a merge mode is applied to a current block; deriving spatial merging candidates for the current block based on the merge flag information indicating that the merge mode is applied to the current block; constructing a merging candidate list including the spatial merging candidates for the current block; deriving motion information of the current block based on one of the spatial merging candidates indicated by the merge index information in the merging candidate list; deriving prediction samples of the current block based on the derived motion information; and generating a reconstructed picture based on the prediction samples, wherein the size of the parallel merging unit region is derived based on the information on the parallel merge level, wherein the current block belongs to the parallel merging unit region, wherein the current block is related to a prediction unit, PU, and the PU is one of PUs partitioned from a coding unit, CU, wherein the PU has a size smaller than the size of the parallel merging unit region and is located within the parallel merging unit region, wherein the spatial merging candidates are identical to spatial merging candidates of a 2Nx2N PU which has a same size as the parallel merging unit region, wherein the spatial merging candidates of the 2Nx2N PU which has the same size as the parallel merging unit region, are derived from a lower left corner neighboring block, a left neighboring block, an upper right corner neighboring block, an upper neighboring block and an upper left corner neighboring block of the parallel merging unit region, and wherein the information on the parallel merge level is received through a picture parameter set. receiving information on a parallel merge level which represents a size of a parallel merging unit region and merge index information; obtaining merge flag information which represents whether a merge mode is applied to a current block; deriving spatial merging candidates for the current block based on the merge flag information indicating that the merge mode is applied to the current block; constructing a merging candidate list including the spatial merging candidates for the current block; deriving motion information of the current block based on one of the spatial merging candidates indicated by the merge index information in the merging candidate list; deriving prediction samples of the current block based on the derived motion information; and generating a reconstructed picture based on the prediction samples, wherein the size of the parallel merging unit region is derived based on the information on the parallel merge level, wherein the current block belongs to the parallel merging unit region, wherein the current block is related to a prediction unit, PU, and the PU is one of PUs partitioned from a coding unit, CU, wherein the PU has a size smaller than the size of the parallel merging unit region and is located within the parallel merging unit region, wherein the spatial merging candidates are identical to spatial merging candidates of a 2Nx2N PU which has a same size as the parallel merging unit region, wherein the spatial merging candidates of the 2Nx2N PU which has the same size as the parallel merging unit region, are derived from a lower left corner neighboring block, a left neighboring block, an upper right corner neighboring block, an upper neighboring block and an upper left corner neighboring block of the parallel merging unit region, and wherein the information on the parallel merge level is received through a picture parameter set.