The invention discloses video encoding and decoding methods using a quad tree, and a decoding device. The decoding method comprises the following steps of: decoding unified code block flag information in a code unit; decoding a split information flag on the basis of the unified code block flag information and information on the size of the code unit; and decoding code block flag information of the code unit if the code unit is not to be split further on the basis of the split information flag. In the step of decoding the split information flag, the split information flag is not to be decoded if a transform coefficient of the code unit does not exist.