1. A display device for displaying a two-dimensional (2D) or a three-dimensional (3D) image, the display device comprising: an image display determination unit which determines whether an input image sequence having a first frame rate is a 2D image sequence or a 3D image sequence; a 2D frame rate conversion unit which receives the input image sequence and generates a 2D output image sequence having a second frame rate if the image display determination unit determines that the input image sequence is the 2D image sequence, wherein the 2D output image sequence comprises the input image sequence and a 2D intermediate image generated from the input image sequence; and a 3D frame rate conversion unit which receives the input image sequence, and generates a 3D output image sequence having a third frame rate if the image display determination unit determines that the input image sequence is the 3D image sequence, wherein a left-viewpoint intermediate image, a right-viewpoint intermediate image and the input image sequence are repeatedly included in the 3D output image sequence, the left-viewpoint intermediate image is determined from at least one left-viewpoint image in a left-viewpoint image sequence included in the input image sequence, and the right-viewpoint intermediate image is determined from at least one right-viewpoint image in a right-viewpoint image sequence included in the input image sequence.