A mask information generator 4 carries out the following processing: (1) defining lines which become respective sides of a polygon of which three or more points on a display screen designated by user's operation become vertices; (2) calculating a value of a coordinate in a vertical direction on each of the lines which has a value of a coordinate in a horizontal direction of each pixel designated by a counter which designates a coordinate of each pixel in an image signal based on a synchronization signal thereof; and (3) comparing a value of the coordinate designated by the counter in the vertical direction of each pixel with the calculated value of the coordinate on each of the lines, to determine whether or not each pixel in the image signal is included in a mask region which is a region within the polygon surrounded by the lines. A masking unit 5 carries out masking using a mask image with respect to each pixel included in the mask region.