determining whether a merge mode is applied to a coding block or not;

in response to the merge mode being applied to the coding block, constructing a merge candidate list for the coding block, the merge candidate list comprising spatial merge candidates and a temporal merge candidate, the spatial merge candidates being derived from spatial neighboring blocks adjacent to the coding block, the temporal merge candidate being derived from a collocated block in a collocated picture;

obtaining motion information of the coding block;

and

obtaining a prediction block of the coding block based on the motion information of the coding block,

wherein in response to a number of merge candidates included in the merge candidate list being less than a first threshold value, an average merge candidate is newly added to the merge candidate list,

wherein the average merge candidate is derived by using a first merge candidate and a second merge candidate which are included in the merge candidate list,

wherein a motion vector of the average merge candidate is derived by averaging a motion vector of the first merge candidate and a motion vector of the second merge candidate, and

wherein a reference picture index of the average merge candidate is inherited from one of the first merge candidate and the second merge candidate who has a merge index less than the other of the first merge candidate and the second merge candidate.