An apparatus for manufacturing a glass-base-material, which is a base material of an optical fiber, includes a driving unit that drives a glass rod around an axis of the glass rod; a burner for accumulating glass soot around outside surface of the glass rod, which is driven by the driving unit, to form the glass-base-material; a weight-deducting unit, on which the driving unit is mounted, for deducting a predetermined weight from a total weight of the driving unit and the glass-base-material formed by the burner; and a measuring unit provided under the weight-deducting unit for measuring the total weight, from which the predetermined weight is deducted by the weight-deducting unit.