The present invention provides a novel acrylic block copolymer rich in flexibility and excellent in mechanical strength, moldability, oil resistance, heat resistance, thermal decomposition resistance, weather resistance, and compression set, and further rich in reactivity. The present invention also provides compositions, seal products, and automobile, electric, and electronic parts, all of which include the acrylic block copolymer. The acrylic block copolymer includes a methacrylic polymer block (a) and an acrylic polymer block (b), at least one of the polymer blocks containing, in its main chain, at least one acid anhydride group (c) represented by formula (1): ##STR00001## (wherein R.sup.1s each represent hydrogen or a methyl group and may be the same or different, n represents an integer of 0 to 3, and m represents an integer of 0 or 1).