A gas sensor has a structure in which a sensor body is secured to a sensor-mounting member using an attachment screw. The gas sensor is capable of ensuring the stability of installation of a protective cover. The gas sensor includes the sensor body in which a sensor device is disposed and the cylindrical attachment screw disposed on an outer circumference of the sensor body to be rotatable. The gas sensor is secured to the sensor-mounting member which has an internal thread engaging the attachment screw and a bearing surface disposed on a front end side of the internal thread. The sensor body has a flange which protrudes outwardly on the front end side of the attachment screw. The flange held between the bearing surface of the sensor-mounting member and the attachment screw in the axial direction. A protective cover is secured to the attachment screw closer to the base end side than an external thread engaging the internal thread is. The protective cover covers an outer periphery of at least a portion of the sensor body which is closer to the base end side than the sensor-mounting member is.