Gas sensor having lead wires extending in insulating and hermetically sealed holder

In a gas sensor having a tubular metal casing, a rod-shaped holder of a ceramic material formed with a plurality of axial through-holes and tightly fitted in the tubular casing, a gas sensing element which operates on an electrochemical principle and which is fixed to a forward end of the rod-shaped holder, a plurality of lead wires which are attached to the gas sensing element and respectively passed through the through-holes in the rod-shaped holder, and a heat-resistant sealant filled into the through-holes in the holder,

• the improvement comprising a rod-shaped plug of a ceramic material, said plug being pre-formed with a plurality of axial through-holes and tightly fitted in an end portion of said tubular casing so as to leave a space between the rear end of said holder and the forward end of said plug, said lead wires being respectively passed through said through-holes in said plug across said space, and a hermetic sealant which fills said space between said holder and said plug and said through-holes in said plug, said hermetic sealant comprises a lead glass which is solidified from a molten state within said space and said through-holes in said plug.