In order to reduce the flicker and blur of a result obtained by, for example, converting an interlace scan image into a progressive scan image, an image still area determination device comprises a unit for calculating the average value of the values of the relevant pixel and pixels adjacent to the pixel in each of the current frame image and a frame image immediately before it, a unit for calculating a difference between the average values of a pair of pixels located in the same position of each of two frame images and a unit for determining a still area in each of the two frame images, using its output and a predetermined threshold value.