We show an unconscious self-assembling ability of block copolymers, that is, spontaneous perpendicular orientation of nanocylinders in a block copolymer thick film of polystyrene-block-polyethylenebutylene-block-polystyrene (SEBS) triblock copolymer. The perpendicular orientation of cylinders was obtained when the SEBS sample underwent morphological transition from spheres to cylinders, which was mediated by directional coalescence of: spheres normal to the film sample. The direction of the cylinder orientation is determined already by the direction of the solvent evaporation, which can be referred to as an effect of a chemical potential gradient, We have not been aware Of Such self-assembling ability of block copolymers. Its robustness can be identified in a thick film (about 1 mm thickness), and it is expected that this opens Lip a wide use of block copolymers as a self-sustaining film in many practical applications.