PURPOSE: To maintain the brightness of a circular fluorescent lamp uniform along the axial direction of a glass tube body.

CONSTITUTION: Electrode sections 2 and 2 are sealed at both ends 3 and 7 of a glass tube body 1. The winding of the body 1 under heating is started from the end 3, while an exhaust tube body 4 at the end 3 is being held, and ended at the end 7 via a sequential process. The body 1 is thereby molded into a circular form. In this case, the minimum value DAm of tube diameter at a position A near one end is made larger than the minimum value DBm of tube diameter at a position B near the other end. According to this construction, the axial elongation of the tube body 1 at the A position can be restrained, and a brightness drop at the position A due to elongation can be prevented at the time of molding the body 1 into the annular form.