Abstract: In fabricating a semiconductor device, a hydrogen-containing first insulating film is formed over a semiconductor layer, a gate insulating film and a gate electrode, and a first heat-treatment in a hydrogen atmosphere is performed. A second insulating film can be formed on the first insulating film, and a second heat-treatment in a hydrogen atmosphere performed. A hydrogen-containing third insulating film can be formed on the second insulating film, and a third heat-treatment in an atmosphere containing hydrogen or nitrogen performed. By these methods, damages to a semiconductor layer caused by hydrogenation can be avoided.