A video-image selection unit cyclically acquires at least two types of video images. A replacing mask determination unit selects, for each acquired video image, a mask pattern for masking a video image acquired by the video-image selection unit by replacing some pixels composing the video image with pixels of a single color such that said some pixels are spatially removed. Based on the mask pattern, the video-image replacing unit replaces said some pixels composing the video image with the pixels of a single color. A video-image output unit outputs the video image for which the replacement was performed. When selecting a mask pattern, the replacing mask determination unit selects a mask pattern that is different from a mask pattern used for the last video image, the mask pattern being different in the position of pixels not to be replaced with pixels of a single color.