1. An inter prediction method comprising: generating a merging candidate list for a current block when a merge mode is applied to the current block; deriving motion information of the current block based on one of a plurality of merging candidates constituting the merging candidate list; and generating a prediction sample of the current block based on the derived motion information, wherein the current block is a prediction unit (PU) belonging to a parallel merging unit, wherein the PU is partitioned from a coding unit (CU), wherein PUs, in the CU and the parallel merging unit, share the same merge candidate list, and wherein the parallel merging unit is determined based on a parallel merge level which indicates a size of the parallel merging unit, and information on the parallel merge level is received through a picture parameter set.