A high definition display unit which demodulates, decodes, and processes in relation to chroma and displays a television signal in which a PI (picture) signal and an SI (service information) signal are mixed. A changeover switch 13 connected to an image quality adjusting circuit in a video chroma processing circuit 7 is opened or closed according to an on-off signal SI showing the superposed period of both signals detected by a decoder 4. If the above on-off signal SI is on, the changeover switch 13 is opened and the adjustment of image quality is disabled. Therefore, SI display never disappears due to the adjustment of image quality, and the quality of an image can be also adjusted according to an ambient status. Other approaches are also disclosed from preventing image quality adjustment of at least portions of a displayed image containing an information image.