To provide a method of forming glass tubes, by which the side parts of parallelly arranged two or more glass tubes are airtightly melt-bonded to each other through communicating tubes without contacting and melt-bonding under pressure regardless of the internal diameter of glass tubes.

The method of forming the glass tubes is a method for airtightly melt-bonding the side parts of parallelly arranged two or more glass tubes to each other through communicating tubes. The method comprises softening the side part of each glass tube by heating it, putting each glass tube into a mold, subjecting the glass tube to blow molding to form a projected part in the side part of the glass tube, taking the glass tube having the formed projected part out of the mold, heating the glass tube while blowing a fluid into it so as to open the projected part and to make the projected part having a partial opening part, heating the projected part having the partial opening part so as to make the projected part having a desired opening area, bringing together the peripheral edge parts of the opening parts of the projected parts to each other, and forming a communicating tube by melt-bonding the peripheral edge parts brought together.