An airway adaptor includes: a nasal expiration collecting portion which is to be connected to a nostril of a living body, and which collects an expiration of the nasal cavity; a gas passage portion that forms a gas passage through which the expiration collected by the nasal expiration collecting portion passes; and a gas sensor attaching portion to which a gas sensor that is disposed in the gas passage portion, and that measures the concentration of a predetermined gas component contained in the expiration is attachable. The nasal expiration collecting portion shows a flowing state where a flow of the expiration is allowed, and an obstruction state where the flow of the expiration is blocked.