Method of manufacturing arc tube

A method of manufacturing an arc tube, the arc tube including a light emitting tube portion, a first pinch seal portion, and a second pinch seal portion, wherein the light emitting tube portion, the first pinch seal portion, and the second pinch seal portion are formed on a quartz glass tube, and wherein the first pinch seal portion and the second pinch seal portion are disposed on both sides of the light emitting tube portion, said method comprising the steps of:

• providing the quartz glass tube substantially vertically with the first pinch seal portion down;
• providing a thermal insulating plate in a boundary position between the light emitting tube portion and a portion to be pinch-sealed in an outer peripheral space of the quartz glass tube;
• jetting liquid nitrogen from a cooling nozzle provided obliquely below the light emitting tube portion toward the light emitting tube portion, thereby cooling the light emitting tube portion, and heating the portion to be pinch-sealed with a burner;
• and
• pinch-sealing the portion to be pinch-sealed with a pincher immediately thereafter,
• wherein the thermal insulating plate includes an upward taper at a portion in the vicinity of an outer periphery of the quartz glass tube.