A gas sensor comprising: a sensor device which detects a concentration of a given gas component of measurement gas; a device-side insulator porcelain which retains the sensor device therein; an atmospheric side insulator porcelain (32, 320, 330, 340) which covers a base end side of the sensor device to define an air atmosphere; a housing which retains said device-side insulator porcelain and said atmospheric side insulator porcelain (32, 320, 330, 340) therein; a seal which is made of a sealing member disposed between an inner peripheral surface of the housing and an outer peripheral surface of the device-side insulator porcelain; and an atmospheric side cover which is secured to a base end side of said housing, wherein said atmospheric side cover includes a large-diameter portion which is formed on a front end side thereof, a small-diameter portion which is formed closer to the base end side than the large-diameter portion is and has a diameter smaller than that of the large-diameter portion, a shoulder portion which is formed between the small-diameter portion and the large-diameter portion to press a base end surface of the atmospheric side insulator porcelain (32, 320, 330, 340) to the front end side through a biasing member (700, 730) to place the atmospheric side insulator porcelain (32, 320, 330, 340) in contact with the device-side insulator porcelain, and an air hole which is formed in a base end portion, wherein an outer space is formed between an outer peripheral surface of the atmospheric side insulator porcelain (32, 320, 330, 340) and an inner peripheral surface of the large-diameter portion, the outer space being arranged adjacent to the seal, wherein an outside path is formed inside the small-diameter portion in communication with the air hole, and wherein the shoulder portion includes a contact portion which contacts the biasing member (700, 730) and a detached portion which is separate from the biasing member (700, 730), a communication path being formed between the detached portion and the biasing member (700, 730) to establish a gaseous communication between the outer space and the outside path, characterized in that the biasing member (700, 730) is an annular elastic member along the shoulder portion and has formed therein i) a plurality of cut-out portions formed in an inner circumferential surface of the biasing member, or ii) through holes to define a gas drain portion leading to said communication path. wherein said atmospheric side cover includes a large-diameter portion which is formed on a front end side thereof, a small-diameter portion which is formed closer to the base end side than the large-diameter portion is and has a diameter smaller than that of the large-diameter portion, a shoulder portion which is formed between the small-diameter portion and the large-diameter portion to press a base end surface of the atmospheric side insulator porcelain (32, 320, 330, 340) to the front end side through a biasing member (700, 730) to place the atmospheric side insulator porcelain (32, 320, 330, 340) in contact with the device-side insulator porcelain, and an air hole which is formed in a base end portion, wherein an outer space is formed between an outer peripheral surface of the atmospheric side insulator porcelain (32, 320, 330, 340) and an inner peripheral surface of the large-diameter portion, the outer space being arranged adjacent to the seal, wherein an outside path is formed inside the small-diameter portion in communication with the air hole, and wherein the shoulder portion includes a contact portion which contacts the biasing member (700, 730) and a detached portion which is separate from the biasing member (700, 730), a communication path being formed between the detached portion and the biasing member (700, 730) to establish a gaseous communication between the outer space and the outside path, characterized in that the biasing member (700, 730) is an annular elastic member along the shoulder portion and has formed therein i) a plurality of cut-out portions formed in an inner circumferential surface of the biasing member, or ii) through holes to define a gas drain portion leading to said communication path.