1. A method of converting two-dimensional (2D) video contents into three-dimensional (3D) video contents by using a content converting apparatus, the method comprising: determining an object to be extracted from a plurality of frames which include 2D contents; determining, for each of the plurality of frames, a respective possession degree of information about the object to be extracted; selecting at least one key frame from among the plurality of frames based on the determined possession degrees of information about the object to be extracted; extracting the object from the selected at least one key frame; assigning a depth for conversion into 3D contents to the extracted object; and performing tracking upon all of the plurality of frames except for the selected at least one key frame.