A gas sensor is proposed, which can detect a gas by a novel configuration while reduction in size is achieved. The gas sensor (1) does not need a light absorption path as in a prior art so that the size can be reduced correspondingly. Further, in the gas sensor (1), a gas is absorbed in an ionic liquid (IL), and a dielectric constant of the ionic liquid (IL) that changes by absorbing the gas can be measured according to a change in light intensity that occurs by a surface plasmon resonance phenomenon in a metal layer (7). Thus, the gas sensor (1) including the novel configuration that can detect a gas based on the change in the light intensity can be realized.