A method for manufacturing a thin film transistor having high electric characteristics with high productivity. In the method for forming a channel region of a dual-gate thin film transistor including a first gate electrode and a second gate electrode which faces the first gate electrode with the channel region provided therebetween, a first microcrystalline semiconductor film is formed under a first condition for forming a microcrystalline semiconductor film in which a space between crystal grains is filled with an amorphous semiconductor, and a second microcrystalline semiconductor film is formed over the first microcrystalline semiconductor film under a second condition for promoting crystal growth.