1. A method of generating multimedia data, in which a multimedia scene comprises media objects based on a scene tree including nodes corresponding to the media objects, the method comprising: generating by a computer a scene tree including a plurality of nodes by allocating a plurality of object information to the plurality of nodes and linking the plurality of nodes to one another based on the relationship between the plurality of object information allocated to the plurality of nodes; adding a null node to the scene tree and allocating to null node first object information regarding an exchangeable media object from among the plurality of object information; and recording the null node in a scene descriptor area in a scene description language, wherein an object-oriented multimedia encoding device encodes the scene tree using the scene description language to form the multimedia data, and the scene description language describes object information regarding the media objects allocated to the nodes in the scene tree, wherein the adding of the null node comprises: defining at least one of an image, which is to be displayed when a media source is not designated by the null node, and an activation indicator indicating whether an event that is to be executed with the object information allocated to the null node is to be activated, in the first object information allocated to the null node.