Disclosed is a carbon nanotube field effect transistor which stably exhibits excellent electrical conduction properties. Also disclosed are a method for manufacturing the carbon nanotube field effect transistor, and a biosensor comprising the carbon nanotube field effect transistor. First of all, an silicon oxide film is formed on a contact region of a silicon substrate by an LOCOS method. Next, an insulating film, which is thinner than the silicon oxide film on the contact region, is formed on a channel region of the silicon substrate. Then, after arranging a carbon nanotube, which forms a channel, on the silicon substrate, the carbon nanotube is covered with a protective film. Finally, a source electrode and a drain electrode are formed, and the source electrode and the drain electrode are electrically connected to the carbon nanotube, respectively. A field effect transistor manufactured by these processes stably exhibits excellent electrical conduction properties since the carbon nanotube, which serves as the channel, is not contaminated.