1. An image processing method, comprising: calculating, by one or more processors, a synthesis ratio based on a motion estimation reliability, said synthesis ratio being used when a first progressive image generated from an interlaced image at a time and a second progressive image generated from plural interlaced images at times different from said time are synthesized, said motion estimation reliability being decided by integrating a first motion estimation reliability, a second motion estimation reliability, and a third motion estimation reliability, said first motion estimation reliability being decided by a difference absolute value between pixels within a preceding interlaced image at a preceding time to said time and pixels within a subsequent interlaced image at a subsequent time to said time, used to calculate a pixel value interpolated when said second progressive image is generated, said second motion estimation reliability being decided by the widths of areas of the plural interlaced images at the preceding and subsequent times used to calculate said pixel values to be interpolated, and said third motion estimation reliability being decided by a combination of a line luminance change of the interlaced image at the preceding time and a line luminance change of the interlaced image at the subsequent time within said area.