To provide a laser apparatus and a laser annealing method with which a crystalline semiconductor film with a larger crystal grain size is obtained and which are low in their running cost. A solid state laser easy to maintenance and high in durability is used as a laser, and laser light emitted therefrom is linearized to increase the throughput and to reduce the production cost as a whole. Further, both the front side and the back side of an amorphous semiconductor film is irradiated with such laser light to obtain the crystalline semiconductor film with a larger crystal grain size.