Provided is a solar cell including: a transparent electrode (2) formed as an n-type semiconductor; a plurality of carbon nanotube groups (3) placed in parallel to each other on and perpendicularly to the lower surface of the transparent electrode (2); and metal electrodes (4) placed on the lower surfaces of the carbon nanotube groups (3) opposite to the electrode (2). The diameters of the carbon nanotubes of the carbon nanotube groups (3) in parallel to each other are varied from one side of the electrode (2) to the other side thereof, and the group III atoms are doped into the carbon nanotube groups to form p-type semiconductors.