1. An image coding apparatus which divides a coding target image into a plurality of blocks, detects a motion vector for each block using a reference image, and performs coding of each block based on the motion vector, comprising: filter unit adapted to apply low-pass filter processing to the coding target image and the reference image as M (M>2)-arized images; N-arization processing unit adapted to convert a first N (M>N≧2)-arized image corresponding to the coding target image and a second N-arized image corresponding to the reference image by respectively N-arizing the coding target image and the reference image which have undergone the filter processing; first motion vector detection unit adapted to detect a motion vector using the second N-arized image for each of the plurality of blocks which form the first N-arized image; second motion vector detection unit adapted to detect a motion vector using the corresponding coding target image and the reference image for a block of the coding target image corresponding to the detection result of said first motion vector detection unit; motion compensation unit adapted to generate a motion-compensated image from the reference image using the detection result of said second motion vector detection unit; and coding processing unit adapted to apply coding processing to a difference between the coding target image and the motion-compensated image.