A display device includes a display unit, an external device interface unit configured to receive video having a second frequency from a set-top box converting an original video having a first frequency into the video having the second frequency, and a control unit configured to: determine whether an abnormal output state is detected, based on the video having the second frequency; when the abnormal output state is detected, determine the number of interpolation frames to be generated so as to output the original video having the first frequency as final video having a third frequency through the display unit; and generate interpolation frames as many as the determined number of interpolation frames.