A gas sensor comprising: a sensor element having a free end and a base end remote from the free end; an inner protective cover having a bottomed cylindrical shape and having a side surface and a bottom surface, the inner protective cover having at least one first inner gas hole and at least one second inner gas hole (138a, 239a, 240a, 241a) formed therein and covering said free end of the sensor element, each said second inner gas hole (138a, 239a, 240a, 241a) being formed in said side surface or said bottom surface of the inner protective cover; and an outer protective cover having a bottomed cylindrical shape, the outer protective cover having a plurality of outer gas holes formed therein and covering the inner protective cover; wherein the first inner gas hole is located so as to be closer to said base end of the sensor element than the second inner gas hole (138a, 239a, 240a, 241a), and the second inner gas hole (138a, 239a, 240a, 241a) is located further from said base end of the sensor element than said free end thereof; wherein a ratio A2/A1 defined by a ratio of an opening area A1 per first inner gas hole and an opening area A2 per second inner gas hole (138a, 239a, 240a, 241a) is higher than or equal to 0.9 and lower than or equal to 3.8, wherein the outer protective cover is a cover having a side surface and a bottom surface, wherein each of the outer gas holes is formed in a corner portion which is a boundary portion between the side surface and the bottom surface of the outer protective cover, and wherein an angle θ1 formed by an external opening plane of the outer gas hole and the bottom surface of the outer protective cover is in the range from 10° to 80°. a sensor element having a free end and a base end remote from the free end; an inner protective cover having a bottomed cylindrical shape and having a side surface and a bottom surface, the inner protective cover having at least one first inner gas hole and at least one second inner gas hole (138a, 239a, 240a, 241a) formed therein and covering said free end of the sensor element, each said second inner gas hole (138a, 239a, 240a, 241a) being formed in said side surface or said bottom surface of the inner protective cover; and an outer protective cover having a bottomed cylindrical shape, the outer protective cover having a plurality of outer gas holes formed therein and covering the inner protective cover; wherein the first inner gas hole is located so as to be closer to said base end of the sensor element than the second inner gas hole (138a, 239a, 240a, 241a), and the second inner gas hole (138a, 239a, 240a, 241a) is located further from said base end of the sensor element than said free end thereof; wherein a ratio A2/A1 defined by a ratio of an opening area A1 per first inner gas hole and an opening area A2 per second inner gas hole (138a, 239a, 240a, 241a) is higher than or equal to 0.9 and lower than or equal to 3.8, wherein the outer protective cover is a cover having a side surface and a bottom surface, wherein each of the outer gas holes is formed in a corner portion which is a boundary portion between the side surface and the bottom surface of the outer protective cover, and wherein an angle θ1 formed by an external opening plane of the outer gas hole and the bottom surface of the outer protective cover is in the range from 10° to 80°.