PURPOSE: To provide an amphipathic elastomeric block copolymer useful for obtaining a lithographic plate excellent in developing, ink absorbency, etc., by forming a copolymer containing specified blocks under specified conditions.

CONSTITUTION: This copolymer contains at least 20 wt.% hydrophobic soft blocks (A) having a glass transition temperature (Tg) of below -30°C, preferably formed from a conjugated diene (e.g. isoprene), 30-60 wt.% hydrophilic hard blocks containing acidic groups, having a Tg of above +30°C, preferably formed from an α,β-ethylenically unsaturated carboxylic acid (e.g. methacrylic acid), 0-79 wt.% hydrophobic soft blocks having a Tg of above +50°C, preferably formed from an aromatic vinyl compound (e.g. styrene), and fundamentally branched or crosslinked homopolymer blocks obtained from units comprising a compound containing at least two vinyl or isopropenyl groups (e.g. trimethyloporpane trimethacrylate), wherein the weight percentage of component A is specified.