A method of accurately detecting rapid motion as well as slow motion using two field memories in an interlaced-progressive scanning converter for changing an interlaced-scanned signal into a progressive-scanned signal. A threshold coefficient, which is used for comparison to a difference between temporal interpolation and spatial interpolation values to determine whether a picture is in motion, is determined dynamically in the disclosed method, rather than being a fixed, predetermined value. The threshold coefficient is determined based upon a degree of correlation between data of pixels above and below a pixel to be interpolated. Detection of whether the picture is stationary is based upon the difference between the data of pixels above and below the pixel to be interpolated is greater than a stationary coefficient. The threshold coefficient is maintained as being no less than the value of the stationary coefficient.