1. An image processing apparatus, comprising: correlation value calculation means for setting a plurality of reference blocks having a size equal to that of a target block formed with a predetermined size in a target screen image and formed from a plurality of pixels within a search range set in a reference screen image different from the target screen image and determining a correlation value between the target block and each of the reference blocks; means for determining the highest value from among the correlation values calculated by the correlation value calculation means and a maximum value from among the correlation values except the highest value; motion vector detection means for detecting a motion vector of the target block as a displacement amount of the reference block with regard to which the highest value of the correlation value is calculated from the target block; and means for calculating the difference between the highest value of the correlation value and the maximum value of the correlation values except the highest value as an index to reliability of the motion vector detected by the motion vector detection means.