Provided is a polyester film for use in the electric insulation of a motor, which has excellent wet heat resistance and heat resistance, can bear long-term use under high-temperature and constant-humidity conditions, and also has excellent processability. A biaxially oriented polyester film satisfying the following requirements (1) to (3):(1) a polyester resin composition that constitutes the film contains organic particles, wherein the content of the organic particles is 1 to 5% by weight inclusive relative to the whole amount of the polyester resin composition that constitutes the film; (2) each of the organic particles does not change the state thereof in a temperature range from 25 to 290 DEG C inclusive and has an aspect ratio (longer diameter/shorter diameter) of 2 to 7 inclusive and a longer diameter of 2 to 10 [mu]m inclusive; and (3) the film has a porosity of 5% or less.