1. A detecting method of a contour of a moving region of a dynamic image for distinguishing a moving object positioned in the moving region, comprising the steps of: inputting a series of frames of dynamic image which consists of variable densities F(x,y,t) of pixels; calculating a variable density gradient fx=.differential.F(x,y,t)/.differential.x defined as an x directional gradient of the variable densities F(x,y,t) for each of the pixels; calculating a variable density gradient fy=.differential.F(x,y,t)/.differential.y defined as a y directional gradient of the variable densities F(x,y,t) for each of the pixels; calculating a variable density gradient ft=.differential.F(x,y,t)/.differential.t defined as a time-based variation of the variable densities F(x,y,t) for each of the pixels; calculating covariances Sxx, Sxy, Sxt, Syx, Syy, Syt, Stx, Sty and Stt according to an equation EQU Sij=.intg..intg..intg.(fi*fj)dxdydt (i,j.epsilon.{x,y,t}) for each of the pixels by utilizing the variable density gradients fx, fy and ft, integration ranges being limited in a space-time region of an observed pixel selected from the pixels and another space-time region of neighboring pixels positioned near to the observed pixel; extracting first, second, third and fourth inherent characteristics of the dynamic image by utilizing the covariances Sxx, Sxy, Sxt, Syx, Syy, Syt, Stx, Sty and Stt, the first inherent characteristic being utilized to judge whether or not an observed pixel selected from the pixels is positioned in a texture drawn in the dynamic image, the second inherent characteristic being utilized to judge whether the texture in which the observed pixel is positioned is a one-dimensional texture changing in a direction only or a two-dimensional texture changing in an x-y plane, the third inherent characteristic being utilized to judge whether luminous intensity of the observed pixel positioned in the one-dimensional texture changes in a motional change or a non-motional change, and the fourth inherent characteristic being utilized to judge whether luminous intensity of the observed pixel positioned in the two-dimensional texture changes in the motional change or the non-motional change; classifying one or more pixels into first or second classification type by utilizing the first to fourth inherent characteristics, the first classification type (S1-ACR) denoting that an observed pixel selected from the pixels is positioned in the one-dimensional texture and the luminous intensity of the observed pixel changes with time in a non-motional change, and the second classification type (S2-ACR) denoting that the observed pixel is positioned in the two-dimensional texture and the luminous intensity of the observed pixel changes with time in the non-motional change; recognizing first pixels classified into the first classification type and second pixels classified into the second classification type as boundary pixels positioned in a boundary region between the moving region and a static region of the dynamic image and setting a region occupied by the boundary pixels as a candidate region for a contour of the moving region; detecting a contour of the moving region by adopting a line passing through a middle portion of the candidate region as the contour of the moving region; distinguishing a moving object positioned in the moving region of which the contour is detected; and processing the dynamic image in which the moving object distinguished exists to display the dynamic image in a displaying unit or transfer the dynamic image to an external unit.