A polymer and composition useful in forming an insulating film provided with a low permittivity, a high heat resistance, and a high mechanical strength and an insulating film obtained from these and an electronic device having the same are provided. The polymer for forming an insulating film according to the present invention is characterized by being obtained by polymerizing a reactive compound represented by Formula (1). The insulating film according to the present invention is formed using a composition for forming an insulating film including that polymer, has molecular spaces having an average space size of 0.7 nm to 5 nm, and has a permittivity of 2.3 or less. The electronic device according to the present invention has the insulating film. (wherein R1, R2, and R3 are the same or different from each other and respectively represent an organic group having a ring structure; X and Y are the same or different from each other and respectively represent an aromatic organic group having a reactive group; and n represents 0 or 1).