PURPOSE: To simply form a recess with little inner bank quantity and uniform thickness by blowing burner flames to a plane containing the axis of a curved section on the inner periphery of the curved section to be formed with the recess from both directions of the vertical direction, heating and softening it, then cooling and solidifying it.

CONSTITUTION: When a recess 14 is to be formed on the inner periphery side of the curved section 13 of a fluorescent lamp 10, a pair of upper and lower burners 15a and 15b are located vertically to the horizontal surface containing the axis of the curved section 13, i.e., vertically in the figure, tip centers of flames 16a and 16b of the burners 15a and 15b are made nearly coincident with the virtual center O of the recess 14, and the flames 16a and 16b are blown to a pair of bulbs 11a and 11b at the required pressure. Parts of a pair of bulbs 11a and 11b around the virtual center O of the recess 14 are heated and softened in a circular shape, for example, and the heated and softened sections are nearly circularly expanded and recessed to the insides of the bulbs 11a and 11b. The heated and softened sections are shrunk by the surface tension of glass when cooled, and the recess 14 is formed.