A video decoding method for decoding a video bitstream, wherein the video to be decoded comprises a plurality of pictures which are split into a plurality of coding blocks, the method comprising: determining whether to split a current coding block according to triangular partitioning, whereby the coding block is split into two triangular prediction units according to one of the two diagonals of the coding block, each diagonal corresponding to one of two triangular partition types; deriving first motion information regarding the first triangular prediction unit in the coding block and second motion information regarding the second triangular prediction unit in the coding block; and acquiring a reference sample for predicting the coding block on the basis of the first motion information and the second motion information, wherein the first motion information regarding the first triangular prediction unit is derived from a first triangular merge candidate specified by first index information signaled in the bitstream, the first index designating one candidate among a plurality of triangular merge candidates included in a merge candidate list, the second motion information regarding the second triangular prediction unit is derived from a second triangular merge candidate specified by second index information signaled in the bitstream, the second index designating one candidate among the plurality of triangular merge candidates included in the merge candidate list, the method being characterized in that: when a value of the second index information is greater than or equal to a value of the first index information, the second triangular merge candidate has an index equal to the value of the second index information plus one, and when a value of the second index information is smaller than a value of the first index information, the second triangular merge candidate has an index equal to the value of the second index information. determining whether to split a current coding block according to triangular partitioning, whereby the coding block is split into two triangular prediction units according to one of the two diagonals of the coding block, each diagonal corresponding to one of two triangular partition types; deriving first motion information regarding the first triangular prediction unit in the coding block and second motion information regarding the second triangular prediction unit in the coding block; and acquiring a reference sample for predicting the coding block on the basis of the first motion information and the second motion information, wherein the first motion information regarding the first triangular prediction unit is derived from a first triangular merge candidate specified by first index information signaled in the bitstream, the first index designating one candidate among a plurality of triangular merge candidates included in a merge candidate list, the second motion information regarding the second triangular prediction unit is derived from a second triangular merge candidate specified by second index information signaled in the bitstream, the second index designating one candidate among the plurality of triangular merge candidates included in the merge candidate list, the method being characterized in that: when a value of the second index information is greater than or equal to a value of the first index information, the second triangular merge candidate has an index equal to the value of the second index information plus one, and when a value of the second index information is smaller than a value of the first index information, the second triangular merge candidate has an index equal to the value of the second index information. when a value of the second index information is greater than or equal to a value of the first index information, the second triangular merge candidate has an index equal to the value of the second index information plus one, and when a value of the second index information is smaller than a value of the first index information, the second triangular merge candidate has an index equal to the value of the second index information.