The purpose of the invention is to provide: a composition for an organic semiconductor insulating film, which is capable of forming an insulating film that exhibits excellent hydrophobicity and smoothness of the surface, while having excellent electrical stability; and an organic semiconductor insulating film obtained by using the composition for an organic semiconductor insulating film. The present composition contains a polysiloxane and an organic polymer compound. The polysiloxane is a polyhedral silsesquioxane having an oxetanyl group and/or an oxetanyl group containing silicon compound represented by the following formula (1). In the formula (1), each of R1-R3 independently represents a monovalent organic group (provided that at least one of R1-R3 is a monovalent organic group having an oxetanyl group); and each of v, w, x and y independently represents 0 or a positive number (provided that w and at least one of v, x and y are positive numbers).