Method of molding optical element and mold for molding optical element

A mold for molding an optical element, the mold comprising:

• an upper mold having an upper neck portion including an upper surface molding surface and a lower mold having a lower neck portion including a lower surface molding surface, the upper surface molding surface and the lower surface molding surface being separated from each other prior to molding and being configured to further approach each other during molding;
• a side surface mold provided with a hole portion into which the neck portion of the upper mold and the neck portion of the lower mold are to be inserted wherein the side surface mold includes a tapered portion on a lower end surface;
• and
• a groove formed between either an upper end surface or a the lower end surface of the side surface mold and the hole portion of the side surface mold, the groove being configured to introduce an inert gas into the hole portion of the side surface mold, wherein the groove includes an opening in an inner peripheral surface of the hole portion and the groove has one end communicating with the tapered portion of the side surface mold and another end communicating with the hole portion of the side surface mold, and
• the opening is positioned such that prior to molding, the neck portion of the upper mold and the neck portion of the lower mold do not face the opening, and during molding, either the neck portion of the upper mold or the neck portion of the lower mold faces the opening.