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 -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 a 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; wherein the formula (1), R<1 >represents a hydrogen atom and a C1-18 linear or branched aliphatic hydrocarbon group; in the formula (2), R<2 >represents a C1-18 linear or branched aliphatic or aromatic hydrocarbon group; F represents a heteroatom or a heteroatom-containing linking group; Z represents a polymer segment containing at least one selected from an oxygen atom, a nitrogen atom, a halogen atom and an aryl group and having a molecular-weight distribution of 1.0 to 3.0; W represents a group selected from an alcoholic hydroxyl group, a phenolic hydroxyl group, a carboxylic acid group, a carboxylate group, an acid anhydride group, an amino group, an epoxy group, a siloxy group and a mercapto group; n is an integer of 1 to 3 and m is 0, 1 or 2 provided that when n is 2 or 3, Z may be the same or different to each other, and when m is 2, W may be the same or different to each other; and W may be bound to the same or different atom of R<1 >to form a cyclic structure.