A gas sensor element of an air/fuel ratio sensor includes a plurality of through holes formed in an insulating substrate at forward end regions of corresponding electrode pads to which the through holes are connected. In the gas sensor element, the through holes are not formed within the longitudinal rear end regions and center regions of the electrode pads. Since in each of the electrode pads, the region of the electrode pad other than its forward end region occupies a greater area than that of the forward end region, it is easy to bring connection terminals into contact with the regions of the electrode pads other than their forward end regions. Therefore, it is possible to prevent the connection terminals from coming into contact with the through holes, so as to thereby prevent occurrence of an electrical connection failure between the connection terminals and the electrode pads.