NOVELTY - An optical film contains polyethylene naphthalate-type resin (P), thermoplastic resin (S) and polymer (M). The thermoplastic resin is obtained by copolymerizing (in %mass) vinyl cyanide monomer (20-40), aromatic vinylic monomer (40-80) and other monomer (0-20). The polymer (M) is obtained by bonding or graft-coupling copolymer (m1) formed using ethylene and (meth)acrylate-type monomer and copolymer (m2) formed using vinyl cyanide monomer (20-40), aromatic vinylic monomer (40-80) and other monomer (0-20).    USE - Optical film is used for reflection type polarizing plate, brightness improving film (both claimed), phase difference plate, viewing angle control film, quarter wavelength plate and half wavelength plate used in LCD, plasma display, organic electroluminescent display and field emission display for projector and rear projection TV. Can also be used in solar cell, lens, waveguide and optical fiber used in optical communication system, optical exchange system and optical measuring system.    ADVANTAGE - The optical film having excellent polarization characteristics, productivity and brightness is manufactured economically. The optical film having sufficient film strength and uniform thickness is obtained.    DETAILED DESCRIPTION - An optical film contains polyethylene naphthalate-type resin (P), thermoplastic resin (S) and polymer (M), and has sea-island structure containing phase (I) containing polyethylene naphthalate-type resin (P) as main component and phase (II) containing thermoplastic resin (S) as main component. The phase (I) or phase (II) is dispersed phase. The dispersed phase has aspect ratio of 2 or more. The volume ratio of phase (I) and phase (II) is 10/90-90/10. The thermoplastic resin is obtained by copolymerizing (in %mass) vinyl cyanide monomer (20-40), aromatic vinylic monomer (40-80) and other monomer (0-20). The thermoplastic resin has weight average molecular weight of 150000-230000 and average refractive index of 1.545-1.575. The polymer (M) is obtained by bonding or graft-coupling copolymer (m1) formed using ethylene and (meth)acrylate-type monomer and copolymer (m2) formed using vinyl cyanide monomer (20-40), aromatic vinylic monomer (40-80) and other monomer (0-20). The content of polymer (M) is 0.1-6 parts mass with respect to 100 parts mass of polyethylene naphthalate-type resin (P) and thermoplastic resin (S).