A method for manufacturing a semiconductor device is provided, which comprises at least a steps of forming a gate insulating film over a substrate, a step of forming a microcrystalline semiconductor film over the gate insulating film, and a step of forming an amorphous semiconductor film over the microcrystalline semiconductor film. The microcrystalline semiconductor film is formed by introducing a silicon hydride gas or a silicon halide gas when a surface of the gate insulating film is subjected to hydrogen plasma to generate a crystalline nucleus over the surface of the gate insulating film, and by increasing a flow rate of the silicon hydride gas or the silicon halide gas.