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 2 D contents; selecting at least one key frame from among the plurality of frames based on a degree of completeness of a shape of the object contained in the frame; extracting the object from the selected at least one key frame; assigning a depth for conversion into 3 D contents to the extracted object; and performing tracking upon all of the plurality of frames except for the selected at least one key frame.