The formation step of a sealing portion 2 includes the substeps of inserting, into the side tube portion 2, a glass member 70 made of a second glass having a softening point lower than that of a first glass constituting the side tube portion 2; tightly attaching forward and backward portions (A and C) of the glass member 70 to the side tube portion 2 by heating the side tube portion 2 when the glass member 70 is divided into the forward portion, the backward portion, and a central portion under the assumption that the side of the glass member 70 closer to a luminous bulb portion 1 is the forward side, thereby forming a cavity 30 between at least a portion of the central portion (B) and the side tube portion 2; and heating, after the attachment step, a portion including at least the glass member 70 and the side tube portion 2 at a temperature higher than the strain point temperature of the second glass.