A gas sensor that can enhance gas detection sensitivity more than the conventional sensors with a simple configuration is proposed. An electric double layer including a gate insulating layer is formed in an ionic liquid (IL), a change of a state of the gate insulating layer in the ionic liquid (IL) that occurs by absorbing a gas is directly reflected in a source-drain current (Isd) that flows in a carbon nanotube (8). Therefore, the gas detection sensitivity can be enhanced more than in the conventional sensors. Further, since the ionic liquid (IL) can be simply provided on a substrate (2) to be in contact with the carbon nanotube (8) and a gate electrode (7), the configuration that chemically modifies a surface of the carbon nanotube with a plurality of polymers as in the conventional gas sensors is not needed, and the configuration can be simplified correspondingly.