A branched olefin copolymer demonstrating excellent performance in various uses, comprising a building block (A) represented by formula (1) below and a building block (B) represented by formula (2) below, obtained by e.g. a method which involves sequentially conducting (step 1) a step of synthesizing a polar group-containing olefin copolymer (Q) containing at least one functional group (G) selected from a hydroxyl group, a carboxylic acid group, an ester group, an amino group, an epoxy group, a silanol group and an acid anhydride group, (step 2) a step of converting the functional group (G) into a group having an ability to initiate radical polymerization, and (Step 3) a step of installing a polymer segment (Z) by radical polymerization of a monomer essentially comprising carbon-carbon double bond-containing compound (R) containing at least one selected from an oxygen atom, a nitrogen atom, a halogen atom and an aryl group; ##STR1##