A field effect transistor of an embodiment of the present invention includes, a semiconductor substrate containing Si atoms; a protruding structure formed on the semiconductor substrate; a channel region formed in the protruding structure and containing Ge atoms; an under channel region formed under the channel region in the protruding structure and containing Si and Ge atoms, the Ge composition ratio among Si and Ge atoms contained in the under channel region continuously changing from the channel region side to the semiconductor substrate side; a gate insulating film formed on the channel region; and a gate electrode formed on the gate insulating film on the channel region.