A gas sensor including a gas sensor element having a first measurement chamber (16); a first pumping cell (11); a second measurement chamber (18) into which a gas to be measured having a controlled oxygen partial pressure is introduced; and a second pumping cell (13) having a second inner pump electrode (13b) and a second counterpart electrode (13c) pump electrode configured to detect a specific gas component. The second inner pump electrode is made of a material that contains, as a principal ingredient, two kinds of Pt particles having different particle sizes and whose particle size ratio measured by a sedimentation particle-size distribution ranges from 1.75 to 14.2. A mixing ratio between large Pt particles and small Pt particles has a mass ratio of 10/90 to 50/50. A 10 kHz-1 Hz resistance value across the second pumping cell at 600° C. is 150Ω or less.