A biaxially oriented polyester film for a membrane switch, which is made from a polyester containing at least 80 mol % of ethylene-2,6-naphthalene dicarboxylate recurring units and which has an endothermic peak with an endothermic energy of at least 0.4 mJ/mg at a temperature of 110 to 160.degree. C.This film is produced by subjecting a biaxially oriented polyester film produced from the above polyester in accordance with a method known per se to the following steps in the order named:(1) the step of heating the film under no strain at a temperature of 150 to 180.degree. C. for 1 to 5 hours; and(2) the step of heating the film in an unrolled or rolled state at a temperature of 80 to 122.degree. C. for 5 to 200 hours.