A method includes the step of: calculating a synthesis ratio based on a motion estimation reliability being decided by integrating a first motion estimation reliability, a second motion estimation reliability, and a third motion estimation reliability, the 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 the second progressive image is generated, the second motion estimation reliability being decided by the widths of areas of interlaced images at preceding and subsequent times used to calculate the pixel values to be interpolated, the 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 the area.