NOVELTY - Voids-containing polyester film has polyester resin (P) and layer (M) containing thermoplastic resin (A) incompatible with polyester and thermoplastic resin (B) incompatible with polyester as cavitation agent. The relative reflectance of surface(s) of the voids-containing polyester film in wavelength of 560 nm is 97% or more. The surface tension of component (A) measured at 23 degrees C under relative humidity of 65% is 20-28. The total content of components (A, B) in the layer (M) is 15-30 %mass. The mass ratio of component (B) and component (A) is 0.2-1.    USE - Voids-containing polyester film is used for light reflective board and back surface reflective sheet for illumination board and solar cell.    ADVANTAGE - The voids-containing polyester film has excellent film-forming property and reflective characteristics. The stripe-shaped defects generated in the surface of the film are reduced.    DETAILED DESCRIPTION - Voids-containing polyester film has polyester resin (P) and layer (M) containing thermoplastic resin (A) incompatible with polyester and thermoplastic resin (B) incompatible with polyester as cavitation agent. The relative reflectance of surface(s) of the voids-containing polyester film in wavelength of 560 nm is 97% or more. The surface tension of component (A) measured at 23 degrees C under relative humidity of 65% is 20-28. The surface tension of component (B) measured at 23 degrees C under relative humidity of 65% is 28-38. The surface tension of the component (A), component (B) and component (P) measured at 23 degrees C and 65% relative humidity satisfies a relation: (absolute value of difference of surface tension of component (A) and surface tension of component (P)) more than (absolute value of difference of surface tension of component (B) and surface tension of component (P)) more than 5. The total content of components (A, B) in the layer (M) is 15-30 %mass. The mass ratio of component (B) and component (A) is 0.2-1. An INDEPENDENT CLAIM is included for manufacture of voids-containing polyester film, which involves melt-kneading polyester resin (P), thermoplastic resin (A) and thermoplastic resin (B) at temperature (T) satisfying a relation: Tg (B)+100 degrees C more than T more than Tm (P)+5 degrees C, with an extruder at shear rate of 50-500 second-1, and extrusion molding into sheet form, where Tm (B) is melting point of component (B) and Tm (P) is melting point of component (P).