PURPOSE: To prepare inexpensively optical fiber having low transmittance loss, by depositing a glass layer having a given refractive index on an inner wall face of anhydrous quartz tube as a starting material, collapsing it under heating, inserting it a quartz tube for adjustment, drawing them in this state under heating, spinning them.

CONSTITUTION: A glass layer having a reractive index equal to a quartz tube is deposited on the inner wall face of the anhydrous high-purity quartz tube (e.g., inner and outer diameter are about 13 and 15mm, respectively) by chemical vapor deposition method, and a core glass layer having a higher refractive index is further deposited on it. The reaction tube is then collapsed to form the parent material 3 for optical fiber consisting of the glass layers 1 and 2(e.g., D27.5mm, D11.5mm). The parent material 3 is inserted into the quanrtz tube 4 for adjustment, they are drawn in this state, spun in an integrated manner, to give the optical fiber 5(e.g., d29μm, d1125μm) having an adjusted ratio of a core diameter to an outer diameter. Consequently, single mode optical fiber having low OH group absorption loss can be prepared in a short time at low cost.