TECHNOLOGY FOCUS - POLYMERS - Preferred Method: Thermoplastic resin and inorganic particles are melt-kneaded to obtain master pellet. The obtained master pellet and polyester (A1) chosen from ethylene terephthalate or ethylene-2,6-naphthalene dicarboxylate (93 mol% or more) are melt-kneaded. The melt-kneaded product is extruded in sheet form and biaxially-stretched to obtain biaxially-oriented polyester film. The ratio of melt viscosity (Av) of polyester (A1) at temperature (Tc) ( degrees C) and shear rate of 200 second-1 and melt viscosity (Mv) of master pellet is 0.2-1. The melt-kneading is carried out under condition satisfying relation: Av/Mv greater than or equal to (-0.183x (Tc-Tm1)+2.095) and Av/Mv less than or equal to (-0.08x (Tc-Tm1)+2.6), where Tm1 is melting point of thermoplastic resin and Tc is extrusion temperature ( degrees C) during melting film forming. Preferred Film: The polyester film is laminated polyester film having polyester layer (P1) and polyester layer (P2) containing polyester (A2) having ethylene terephthalate, high melting point resin (B2) and inorganic particles (C2). The amount of inorganic particles (C2) in layer (P2) is 0.1-5 %mass. The difference of amount of inorganic particles (C1) in layer (P1) and inorganic particles (C2) in layer (P2) is 5-25 %mass. The amount of thermoplastic resin in layer (P1) is 2-40 %mass. The polyester (A1) and thermoplastic resin satisfy relation: 1.5x MwA1'/MwA1 less than or equal to MwD1'/MwD1, where MwA1' is weight average molecular weight of thermoplastic resin after processing at 125 degrees C in relative humidity of 100% for 72 hours, MwA1 is weight average molecular weight of polyester resin (A1) before processing, MwD1' is weight average molecular weight after processing, and MwD1 is weight average molecular weight before processing. The thermoplastic resin has tan- delta peak temperature of 90-200 degrees C at 1 Hz measured by dynamic viscoelastic measurement, and melt viscosity of 500-15000 poise at 270-320 degrees C and shear rate of 200 second-1. The amount of inorganic particles (C1) is 0.5-30 %mass. 70% or more total number of inorganic particles exist is dispersed phase, or in contact with dispersed phase. The melting point of thermoplastic resin is more than melting point of polyester by 5-60 degrees C. The melting point of thermoplastic resin is 260-320 degrees C. The dispersed phase consists of 1-5 pieces/1000 nm. The average length of major axis direction of dispersed phase is less than 10000 nm. The back sheet has polyester film at outermost layer. At least one outermost layer is layer (P1).