1. An image processing method comprising the step of: calculating 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 certain time and a second progressive image generated from plural interlaced images at a time different from said certain 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 an interlaced image at a preceding time and pixels within an interlaced image at a subsequent 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 interlaced images at preceding and subsequent times used to calculate said pixel values to be interpolated, said third motion estimation reliability being decided by a combination of a line luminance change of an interlaced image at a preceding time and a line luminance change of an interlaced signal at a subsequent time within said area.