1. An apparatus for outputting a private image using a public display, comprising: means for generating at least one private image; means for generating at least one masking image that masks the private image; means for generating an image sequence of the private image and the masking image; and means for outputting the private image and the masking image to the display according to the image sequence; wherein the masking image generating means generates a dynamic inverse image of the private image as the masking image according to a refresh rate of the display and the image sequence; and wherein the masking image generating means calculates a mean image value G ad , and then calculates a dynamic inverse image value G ai by applying the calculated mean image value G ad to the following equation, 
 G ai γm =2 G ad γm −G ap γm or G ai γm =G amax γm +G amin γm −G ap γm where G ai is the dynamic inverse image value, G ad is the mean image value, G ap is a private image value, G amax is a maximum image value, G amin is a minimum image value, and γm is a gamma value of the private image outputting apparatus.