A seal (5) of a bulb which is made of a functionally gradient material prepared by mixing a nonconductive material and a conductive material at a ratio changing in the longitudinal direction continuously or stepwise and whose one end functions as a nonconductive region while the other end functions as a conductive region, wherein at least a part of the surface of the conductive region and/or at least a part of an outer lead (6) protruding from the seal (5) is covered with an air shielding layer (7) to suppress the oxidation of the conductive region and the outer lead, so that the life of the bulb can be extended.
