A control device includes a data storing unit, a synchronization control unit, and a duty control unit. The synchronization control unit starts a frame period when a vertical synchronization signal is received. The frame period includes a light emission period and a light extinction period. The duty control unit controls a length of the light extinction period, based on a ratio between a total number of vertical lines in a current frame and a pre-set minimum number of vertical lines in a video image displayed on a display panel during a frame period, such that a ratio of a length of the light emission period and a length of the light extinction period during the frame period when a video image is displayed with the total number of vertical lines in the current frame is constant.