Patent ID: 7886561

Claim:
A method for elongating a glass parent material, from an original diameter of the glass parent material to a target diameter by input of the glass parent material into a heat source, the method including steps of: measuring an outer diameter of a glass parent material elongating portion; calculating at least one item selected from: a relative input rate of the glass parent material into the heat source, a relative elongating rate of the glass parent material from the heat source, and a heating temperature of the heat source, using an expression of A and at least one of B and C including a coefficient of A, and at least one of coefficients of B and C, where A is a difference between the target diameter and the measured diameter, B is an integral value of A over a predetermined time, and C is a change rate of A per unit time; switching at least one of the coefficients of A, B, and C in the expression from a first value to a second value different from the first value, when at least one of A and C passes over corresponding one of a threshold value of A and a threshold value of C so that the measured outer diameter becomes the target diameter; and heating and elongating the glass parent material such that at least A or C crosses the threshold value.