NOVELTY - A pi -electron conjugation block copolymer having polymer block (A) and polymer block (B) is new. The polymer blocks (A) and (B) contain monomeric unit having at least one condensed-ring ( pi )-conjugated structure including thiophene ring(s) in portion of chemical structure. In polymer blocks (A) and (B), heteroaryl structure comprising principal chain of the monomeric units mutually differs, or number of heteroatoms in substituent in monomeric unit of polymer block (A) is 2 or less and number of heteroatoms in substituent in monomeric unit of polymer block (B) is four or more.    USE - New pi -electron conjugation block copolymer is used for composition for photoelectric conversion element (all claimed). Uses include but are not limited to optical sensor, organic thin-film solar cell, inorganic-type solar cell, optical switch, phototransistor and optical memory.    ADVANTAGE - The pi -electron conjugated block copolymer is capable of increasing amount of optical absorption by photoelectric conversion active layer and controlling the morphology. The element containing the conjugated block polymer has excellent photoelectric conversion efficiency.    DETAILED DESCRIPTION - A pi -electron conjugation block copolymer having polymer block (A) and polymer block (B) is new. The polymer blocks (A) and (B) contain monomeric unit having at least one of condensed-ring ( pi )-conjugated structure including thiophene ring(s) in a portion of chemical structure, and have different monomeric units from each other. In the polymer blocks (A) and (B), heteroaryl structure comprising principal chain of the monomeric units mutually differs, difference in carbon number of substituent in the monomeric units is four or more, or number of heteroatoms in substituent in monomeric unit of polymer block (A) is 2 or less, and number of heteroatoms in substituent in monomeric unit of polymer block (B) is four or more. INDEPENDENT CLAIMS are included for the following:    (1) composition, which contains pi -electron conjugation block copolymer and electron-accepting material; and    (2) photoelectric conversion element, which has a layer containing the composition.