An organic semiconductor thin film exhibiting high mobility and a method for preparing the same, and a material for preparing the organic semiconductor thin film by a wet process and a method for preparing the same are provided. Further, an organic semiconductor device excellent in electronic characteristics is provided. A mixture of high purity pentacene and 1,2,4-trichlorobenzene was heated to prepare a homogeneous solution, and a silicon substrate having a pattern of a gold electrode as a source/drain electrode formed thereon was heated to 100.degree. C. and the resultant pentacene solution was spread on the surface of the substrate. When 1,2,4-trichlorobenzene was evaporated, a pentacene thin film was formed. A transistor was formed with the use of the thin film.