A picture decoding method performed by a decoding apparatus, the method comprising: deriving a context index for a split flag syntax element based on split availability information of a current block; determining a context model based on the derived context index; decoding a value of the split flag syntax element based on context-based adaptive binary arithmetic coding, CABAC, by using the determined context model; deriving a current coding unit from the current block based on the value of the split flag syntax element; deriving a predicted block based on inter-prediction or intra-prediction for the current coding unit; and generating a reconstructed block based on the predicted block, wherein the split flag syntax element comprises: a directional split flag representing whether the current block is to be vertically split, wherein the split availability information comprises: a binary tree, BT, horizontal split available flag information related to whether the current block is allowed to be horizontally split based on a BT structure, a ternary tree, TT, horizontal split available flag information related to whether the current block is allowed to be horizontally split based on a TT structure, a BT vertical split available flag information related to whether the current block is allowed to be vertically split based on the BT structure, and a TT vertical split available flag information related to whether the current block is allowed to be vertically split based on the TT structure, characterized in that the context index for the directional split flag is derived based on a number of horizontal split available flags and a number of vertical split available flags, and the horizontal split available flags include the BT horizontal split available flag information with value 1 and the TT horizontal split available flag information with value 1, and the vertical split available flags include the BT vertical split available flag information with value 1 and the TT vertical split available flag information with value 1. deriving a context index for a split flag syntax element based on split availability information of a current block; determining a context model based on the derived context index; decoding a value of the split flag syntax element based on context-based adaptive binary arithmetic coding, CABAC, by using the determined context model; deriving a current coding unit from the current block based on the value of the split flag syntax element; deriving a predicted block based on inter-prediction or intra-prediction for the current coding unit; and generating a reconstructed block based on the predicted block, wherein the split flag syntax element comprises: a directional split flag representing whether the current block is to be vertically split, wherein the split availability information comprises: a binary tree, BT, horizontal split available flag information related to whether the current block is allowed to be horizontally split based on a BT structure, a ternary tree, TT, horizontal split available flag information related to whether the current block is allowed to be horizontally split based on a TT structure, a BT vertical split available flag information related to whether the current block is allowed to be vertically split based on the BT structure, and a TT vertical split available flag information related to whether the current block is allowed to be vertically split based on the TT structure, characterized in that the context index for the directional split flag is derived based on a number of horizontal split available flags and a number of vertical split available flags, and the horizontal split available flags include the BT horizontal split available flag information with value 1 and the TT horizontal split available flag information with value 1, and the vertical split available flags include the BT vertical split available flag information with value 1 and the TT vertical split available flag information with value 1.