NOVELTY - The film (10) has two or more constant content rate areas (1-3) on both sides of a border region (5,6) whose thickness is 0.15-3 mu m. The difference of content rate of microparticles of pairs of constant content rate areas on the sides of border region is 1-30%. The border region contains microparticles or void whose content rate is within the range of content rate of microparticles or void in the constant content rate areas. The difference of content rate of void of pairs of constant content rate areas on sides of border region is 1-30%.    USE - White multilayer polyester film of laminated film used for backsheet of solar cell module (all claimed).    ADVANTAGE - The partial discharge voltage of the white multilayer polyester film is increased and maintained for a long time by providing two or more constant content rate areas on sides of the border region. The power generation efficiency of the solar cell is improved and formation of cracks in the white multilayer polyester film is prevented. Hence, peeling of the white multilayer polyester film is suppressed and durability of the white multilayer polyester film is improved.    DETAILED DESCRIPTION - The content rate of microparticles or void in the constant content rate areas, and thickness of the border region are set satisfying a predetermined formula. The polyester film is comprised such that content rate of microparticles or void in border region changes with respect to the microparticles or void of constant content rate areas along thickness direction. The average value of carboxylic acid in the polyester film is 1-20 eq/tons. The variation of an in-plane partial discharge voltage of the polyester film is 1-15%. The ratio for which thickness of the constant content rate area accounts with respect to thickness of the whole white multilayer polyester film is 95-99.95% and the ratio for which the thickness of the border region accounts is 0.05-5%. The thickness of the polyester film is 40-300 mu m. The average content rate of the microparticles is 1-20 mass% with respect to the total amount of polyester contained in the white multilayer polyester film. The average content rate of the void is 5-60% with respect to the cross-sectional area of the white multilayer polyester film. The microparticles are made of inorganic fine particles such as titanium oxide. The total amount of polyester containing carbodiimide compound and/or a ketene imine compound is 0.1-10 mass% with respect to the content rate of the carbodiimide compound and/or a ketene imine compound in the white multilayer polyester film. INDEPENDENT CLAIMS are included for the following:    (1) a manufacturing method of white multilayer polyester film, involves performing co-extrusion process using a cast drum to form a film with molten resin in which content rate of microparticles differs. A folding is formed in the interface of the co-extruded molten resin while varying discharge amount of molten resin at the rate of 0.1-5% with respect to the cast drum, The temperature of the molten resin is 0.3-5 degrees C during film formation process.;    (2) a laminated film includes a conductive layer in the surface of the white multilayer polyester film. The specific resistance of the surface of the laminated film is 106-1014 ohm/square. The laminated film comprises a bonding layer whose thickness is 1-30%; and    (3) a manufacturing method of laminated film, involves coating a bonding layer such that thickness of the bonding layer is 1-30%.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the white multilayer polyester film.    Constant content rate areas (1-3)    Border region (5,6)    White multilayer polyester film (10)