On an inner wall of a side wall portion of a funnel, a pair of protrusions are integrally formed as glass buildups at positions separated by a predetermined distance from an anode button toward a side of a neck tube. The pair of the protrusions are formed symmetrically on both sides of a virtual baseline extending to an seal edge surface and the neck tube through the center of the anode button. The virtual baseline is an intersection between a virtual plane including a tube axis and the inner wall, and passes through approximately the center of the seal edge surface in the lengthwise side.