Abstract: A gas sensor includes: a light source that emits a first light of a predetermined wavelength toward a gas subject to detection; a density detector that receives the first light and detects a density of the gas subject to detection based on absorption of the first light by the gas subject to detection; a translucent member provided between the light source and the density detector; a temperature adjustment unit that varies a temperature of the translucent member; and a humidity detection unit that detects a humidity of the gas subject to detection based on variation in an amount of the first light received in the density detector and by using a temperature of the translucent member and an ambient temperature of the gas sensor.