A video decoding method includes, when inter prediction of a current block is performed in an affine merge mode, generating an affine merge candidate list including a control point-based affine merge candidate corresponding to control point motion vectors determined by using motion vectors of neighboring blocks included in a block group of representative neighboring blocks according to corners of the current block, determining an affine motion vector of the current block by using control point motion vectors corresponding to a merge candidate selected from the affine merge candidate list, and obtaining prediction samples of the current block by using the affine motion vector of the current block, wherein when a right block of the current block is available, a representative neighboring block adjacent to a lower right corner of the current block is a block whose available motion information is obtained from among a neighboring block adjacent to the lower right corner of the current block and a right boundary of the current block and a neighboring block diagonally adjacent to the lower right corner of the current block.