A laser processing apparatus capable of imparting heat sealing properties to a biaxially stretched polyester film through a method having high efficiency and high safety. The laser processing apparatus includes a laser oscillator, where a film formed of a single layer of a biaxially stretched polyester or a laminate containing a layer of a biaxially stretched polyester on the surface is irradiated with laser light emitted from the laser oscillator, to impart heat sealing properties to a region of the film irradiated with the laser light. The laser processing apparatus may include an optical element which shapes a spot profile of the laser light into a predetermined profile, and may also include a film mounting part which mounts the film.