A novel fluoropolymer which can be an optical resin material having a low refractive index and excellent heat resistance, and a novel fluorinated diene compound having two unsaturated bonds, capable of presenting such a fluoropolymer, are presented. Further, by virtue of the low refractive index and excellent heat resistance, the polymer presents a high performance optical transmitter and a plastic optical fiber.A fluorinated diene compound represented by CF2═CFCF(ORf)CF2OCF═CF2 (wherein Rf is a perfluoroalkyl group such as a trifluoromethyl group), and a fluoropolymer thereof. Further, an optical transmitter made by using such a fluoropolymer, and a plastic optical fiber having a core comprising such a fluoropolymer and a fluorinated low molecular weight compound contained therein as a refractive index-increasing agent.