To provide a elongating method for elongating an optical fiber parent material with high reliability by implementing a high precision control for the outer diameter in short time. This elongating method includes a process for switching a control method for at least one of the control items of measurement object, based on at least one of a difference between measured value and target value and a change rate of the difference per unit time, in measuring the outer diameter of a glass parent material elongating portion and making a feedback control for the elongating conditions based on the difference from the target outer diameter.