PURPOSE: To prepare the titled base material having small bead length and large index of double refraction by arranging a pair of stress exerting member having equal outside dia. and a pair of quartz bar having equal outside dia. to the stress exterting member to contact with each other and providing a preform to the central part and spacer rods in a quartz tube to the space between the quartz tube, stress exerting members and quartz bars, and collapsing the tube by heating.

CONSTITUTION: A pair of stress exerting member 10 and a pair of quartz bar 11 having the equal outside dia. to the stress exerting member are inserted alternately into a quartz tube 12. The radius of the quartz tube is designated as (a) and the inside radius of the quartz tube is designated as (21/2+1)a, and the thickness of the quartz tube wall is designated as 0.6a. A preform 13 comprising a core part 13a and a clad part 13b is inserted into a space surrounded by the stress exerting member 10 and the quartz bar 11 and spacer rods 14 are inserted to other spaces in the quartz tube. The radius of the spacer rod 14 is designated as (21/2-1)a. Thereafter, each members 10, 11, 12, 13 is heated while evacuating the inside of the quartz tube to collapse the tube to form one body. In this way, a base material 7 for optical fiber having a pair of stress exerting member 6 with an almost sectorial sectional shape formed on the outside periphery of the preform 13 is obtd.