NOVELTY - The (pi)-conjugated polymer (300) comprises a principal chain (200) and at least one side-chain compound (100) coupled to the principal chain consisting of a (pi)-conjugated polymer. At least one of side-chain compound is a (pi) conjugated polymer which is (pi)-conjugated system tree-like branch compound having first to nth generation branch units (1-3) which are branched in the shape of a tree on a core unit (10).    USE - For organic thin film, organic electroluminescent element, solar cell (all claimed), organic FET element and optoelectric transducer.    ADVANTAGE - The conjugated polymer is easily soluble in an organic solvent and provides organic thin film having excellent electric-charge mobility. Reduction of glass transition temperature of the polymer, is suppressed. High-temperature durability of the polymer is improved.    DETAILED DESCRIPTION - The (pi)-conjugated polymer comprises a principal chain (200) and at least one side-chain compound (100) coupled to the principal chain consisting of a (pi)-conjugated polymer. At least one of side-chain compound is a (pi)-conjugated polymer which is (pi)-conjugated system tree-like branch compound (where n which is the generation number of a terminal branch unit, is one or more) obtained by coupling (6) and growing, so that the first to nth generation branch units (1-3) are branched in the shape of a tree on a core unit (10) as center, having a tree-like branch in a side chain. INDEPENDENT CLAIMS are included for the following:    (1) organic thin film equipped with the layer containing (pi)-conjugated polymer having a tree-like branch in a side chain;    (2) organic electroluminescent element equipped with the layer containing (pi)-conjugated polymer having a tree-like branch as an electric charge carrying layer in the side chain; and    (3) a solar cell equipped with the layer containing (pi)-conjugated polymer having a tree-like branch in the side chain.    DESCRIPTION OF DRAWING(S) - The figure shows the typical drawing of (pi)-conjugated polymer having tree-like branch in a side chain.    first generation branch unit (1)    second generation branch unit (2)    third generation branch unit (3)    coupling (6)    core unit (10)    side-chain compound (100)    principal chain (200)    (pi) conjugated polymer (300)