A contact combustion type carbon monoxide sensor having no sensitivity to H.sub.2, C.sub.2 H.sub.5 OH and other gases but having a CO sensitivity of 25 to 50 mV at a bridge voltage of 6 to 12 V and a concentration of 500 ppm. Each coil is made of a Fe.Pd system alloy wire material having a large diameter, a specific resistance value, a Vickers' hardness value and a temperature coefficient values each falling within a specific range. A catalyst comprising Cu.sub.2 O, ZnO, MnO.sub.2 and Pt black is formed on the upper surface of the coil of an active portion, and a catalyst comprising CuO and Cr.sub.2 O.sub.3 is formed on the upper surface of the coil of a dummy portion.