A mercury supplying method capable of always supplying a constant amount of mercury into a chamber part easily by executing measurement and ejection of the mercury at the same time. A small diameter pipe as a mercury supply path, is inserted through a glass tube elongating from a chamber part of an arc tube from above so as to have the tip part thereof substantially reaching into the chamber part, and connecting the upstream side of the sealed mercury supply path (mercury barrel) with a gas pressure adjuster. The gas pressure adjuster keeps the inside of the mercury supply path (mercury barrel) at a predetermined pressure so as to maintain the inside of the small diameter pipe in the state filled with mercury as well as applying a predetermined pressure to the mercury in the small diameter pipe for a predetermined time and thus ejecting a predetermined amount of mercury from the small diameter pipe tip part so as to be dropped and supplied to the chamber part. Measurement and ejection of the mercury can be executed at the same time with the small diameter pipe inserted through the glass tube of the arc tube from above. A predetermined amount of mercury can always be supplied into the chamber part in a short time.