Disclosed are a device and a method for displaying a private image on a public display device. Image sequence pattern is generated for the private image and the corresponding masking image. The masking image is made from the dynamic inverse image of the private image, based on the refresh rate of the display device and the image sequence pattern. The masking image can screen the private image more effectively. The private image and the masking image is displayed on the display device according to the image sequence pattern.