1. An image processing apparatus comprising: an obtaining unit configured to obtain a plurality of images; a calculation unit configured to calculate correlation values between the plurality of images with respect to relative positions; a detection unit configured to detect a motion vector between the plurality of images based on the correlation values; and a determination unit configured to determine reliability of the motion vector, wherein the determination unit determines the reliability based on a value indicating the highest correlation among the correlation values and a correlation value, among the correlation values, obtained at a position that is distant by a predetermined distance or longer from a position at which the value indicating the highest correlation is obtained.