An amperometric CO sensor using a solid oxide electrolyte was studied, and it was found that the oxygen pumping current increased with increasing CO concentration when Au(10 wt%)/In1.9Sn0.1O3 and RuO2(10 wt%) were used for cathode and anode, respectively, and La0.9Sr0.1Ga0.8Mg0.2O3 was used as the electrolyte. In this study, the electrode was prepared by DC sputtering. Although the morphology of the deposited electrode film was dense, the sensor responded sensitively to CO if the electrode was prepared by DC sputtering. Coating RuO2/La0.6Sr0.4CoO3 powder on a RuO2 electrode prepared by sputtering effectively increased sensitivity because of the increased surface area and oxidation activity, and a sensitivity to CO of ca. 19 μA/decade was achieved, which is ca.4 times higher than that of powder painted electrodes (4.5 μA/decade) at 673 K in our previous study [Sens. Actuators, B 223 (2016) 535].