Embodiments of the present invention provide an organic semiconductor excellent in the photoelectric conversion efficiency and also a solar cell using the same. This organic semiconductor has a polymer structure comprising a repeating unit represented by the following formula (I): -[A-D]- (I). In the formula, A is a structure represented by and D is a structure having a benzodithiophene skeleton or the like. In the above, R1 is independently H, a substituted or unsubstituted straight-chain or branched-chain alkyl group, or a substituted or unsubstituted straight-chain or branched-chain alkoxy group. The solar cell according to an embodiment of the present invention comprises an active layer containing the organic semiconductor.