Disclosed is a gas sensor having high detection sensitivity, capable of detecting a gas at a low concentration and achieving lower power consumption. In the gas sensor including a gas detection element disposed on a thin-film insulator layer, the gas detection element includes: a heater layer provided on the thin-film insulator layer; and a gas detection layer having a thin-film thermistor part and an electrode part in contact with the thin-film, thermistor part, the gas detection layer being provided on the heater layer so as to be electrically insulated from the heater layer. The gas sensor is configured to detect a temperature change due to contact of a gas with the gas detection element on the basis of a change in resistance value in the thin-film thermistor part. The thin-film thermistor part preferably comprises a composite metal oxide containing Fe2O3, TiO2 and MgO.