A forming method of a refractive index matching film comprises steps: one end of an optical fiber is brought into contact with a liquid surface of a photo-curable refractive index matching solution; curing reaction light is made incident to the opposite end of the optical fiber and is emitted through the one end of the optical fiber so as to cure preliminarily the photo-curable refractive index matching solution in contact with the one end of the optical fiber; the curing reaction light is tentatively stopped; the one end of the optical fiber is separated from the liquid surface of the photo-curable refractive index matching solution; and the curing reaction light is made incident again from the opposite end of the optical fiber so as to cure the preliminarily cured photo-curable refractive index matching solution and form a refractive index matching film.