Abstract: A probe having the inventive composition, it is possible to measure the activity of the solute carbon which could not be measured by the prior art because there did not exist solid oxide or solid composite oxide which were stable at high temperature.The probe comprises a solid electrolyte having a mixed subelectrode provided on its surface, and a reference electrode for providing a predetermined oxygen potential is disclosed. The mixed sub-electrode coating on the solid electrolyte includes a mixture of a carbide MC and of an oxide MO of the element M other than carbon of which the carbide is constituted.