PURPOSE: To clearly observe a display image by providing a random pattern generating means to prevent an image non-display area fluorescent body from burning and to shield useless regions.

CONSTITUTION: A main picture to be displayed on a main screen is selected. Its aspect ratio is detected, when that picture is laterally long, the image is displayed as it is and when the aspect ratio is standard, the presence/absence of any sub picture to be displayed on a sub screen is discriminated. When there is any sub picture, it is reduced by reduction circuits 46, 48 and 50 and the main picture and the sub picture are synthesized by a picture synthesizing circuit 44 and displayed. When there is no sub picture, a system control circuit 36 selects the output of a fixed pattern generation circuit 32 by using a selecting circuit 30 and displays a fixed pattern in a sub picture display area. Afterwards, a shielding plate 52 is moved up to a limit switch 58 in the direction of a shielding position. When the plate arrives at the shielding position, the circuit 36 stops the plate 52 and the circuit 30 selects the output of a random pattern generation circuit 34 and displays the image of a random pattern in the sub picture display area.