To manufacture glass base material with high manufacturing yield, provided is a glass base material elongating method comprising forming a tapered portion where the outer diameter of the glass base material changes continuously, holding the glass base material with chucks, heating the glass base material held by chucks with a heat source, and with a portion of the glass base material softened, increasing the distance between the chucks to elongate the glass base material. The elongation begins from a state in which a position of the heat source at a position at which the outer diameter of the glass base material is set in a range from no less than 95% to no more than 98% of an average outer diameter of the trunk portion of the glass base material.