PURPOSE: To find the moving vector of the whole image with high accuracy by using the value of a quadratic form having the value calculated from the spatially differentiated value as its coefficients as the value representing reliabil ity in the direction of angle for detected moving vector.

CONSTITUTION: The moving vector (u,v) which minimizes the sum Er or resid ual of equation calculated when equation Ex.v+Ey.v+Et=0 representing the relation of the spatial derivatives Ex, Ey and the time derivative Et of an image E (x,y.t) continued in point of time and the moving vector (u,v) on the image, and a condition that the moving vector (u,v) is kept constant in an area S set on the image are applied to each differentiated value calculated from an actual image is set as the moving vector of the area S on the image. Then, the value of quadratic form represented in equation II in which a value represented in equation I calculated from the spatial differentiated value is set as the coefficient is used as the value representing the reliability in the direction of angle θ of the detected moving vector. In such a way, it is possible to detect a mobile body with high accuracy.