The present invention relates to an organic semiconductor thin film suitably employed in electronics, photonics, bioelectronics, or the like, and a method for forming the same. The present invention further relates to a solution for an organic semiconductor used to form the organic semiconductor thin film and an organic semiconductor device using the organic semiconductor thin film.The transistor of the present invention is manufactured by forming sequentially a gate electrode (2), an insulator layer (3), a source electrode and drain electrode (4, 4) on a glass substrate (5), applying thereto a 0.05% (by mass) solution of pentacene in o-dichlorobenzene and drying the solution to form an organic semiconductor thin film (1).The present invention provides a transistor with superior electronic characteristics because the organic semiconductor thin film (1), which can be formed easily at low cost, is almost free of defects.