1. A method of detecting field motion vector data representative of motion of a partial area of an input field picture from any area of a reference field picture, and of detecting frame motion vector data representative of motion of an input frame picture including a plurality of input field pictures from any area on a reference frame picture, comprising the steps of: detecting a first field motion vector from a position of a partial area of the reference field picture at which distortion between the partial area of the reference field picture and a partial area of a first input field picture has a minimum value; detecting a second field motion vector from a position of a partial area of the reference field picture at which distortion between the partial area of the reference field picture and a partial area of a second input field picture has a minimum value; determining initial data for detecting said frame motion vector data on the basis of said first field motion vector data and said second field motion vector data; and processing said initial data to detect said frame motion vector data from a position of a partial area of the reference frame picture at which distortion between the partial area of the reference frame picture and a partial area of the input frame picture has a minimum value, wherein a position of the partial area of the reference field picture at which said first distortion has a minimum value is determined by setting a position of the partial area of the reference field picture detected for the preceding field picture as its center such that a position of the partial area of the reference field picture at which the first distortion between the partial area of the input field picture and the partial area of the reference field picture has a minimum value is detected.