An inter-prediction method according to the present invention comprises the steps of: deriving a first movement vector of the current block; deriving prediction samples for the current block on the basis of the first movement vector; deriving a subunit located in the current block and adjacent to the target boundary of same; deriving a first reference unit on the basis of the subunit and first movement vector; deriving a second movement vector of a neighboring block adjacent to the target boundary; deriving a second reference unit on the basis of the subunit and second movement vector; and deriving the modified prediction samples on the basis of weighted sum of the first reference unit and second reference unit. According to the present invention, by increasing the effectiveness of inter-prediction at the block boundary, amount of data required for residual data can be reduced and the overall coding efficiency can be increased.