1. An image processing apparatus, comprising: motion vector detection means for detecting a motion vector of each of a plurality of blocks of a predetermined size set in an image and formed from a plurality of pixels; and global motion calculation means for carrying out convergence mathematical operation, from the motion vectors of the blocks detected by said motion vector detection means, using extended affine transformation in which at least one of affine parameters is represented by a polynomial function of a variable regarding a displacement axis of the image to calculate a global motion representative of deformation applied to the entire image, the extended affine transformation correcting oblique distortion of the image, wherein, in initial convergence mathematical operation by said global motion calculation means, ordinary affine transformation in which the affine parameters of fixed coefficients are used is carried out from the motion vectors of the blocks detected by said motion vector detection means, and the convergence mathematical operation in which the extended affine transformation is used is started after the initial convergence mathematical operation, and wherein said global motion calculation means includes: correlation value calculation means for determining correlation values between target blocks corresponding to the blocks set in a target screen image and plural ones of reference blocks corresponding to the blocks set on a reference screen image different from the target screen image which are set within a search range in one of the target blocks; means for determining the highest value of the correlation values calculated by said correlation value calculation means and a maximum value of the correlation values except the highest value; motion vector detection means for detecting a motion vector of the target block as a displacement of the reference block, with regard to which the highest value of the correlation values is calculated, from the target block; and means for calculating a difference between the highest value and the maximum value of the correlation values as an index to reliability of the motion vector detected by said motion vector detection means, said global motion calculation means starting the calculation mathematical operation of the global motion using the motion vector weighted with the index to the reliability of the motion vector.