PURPOSE: To efficiently obtain a coated optical fiber having high quality and tolerance of high accuracy by passing a fiber formed by coating an optical fiber core maintaining a circular cross-section with a flowable jacket through a non-circular opening die to stretch the fiber and curing the jacket after forming the jacket to a non-circular sectional shape.

CONSTITUTION: The optical fiber core 42 having the circular cross section is coated with the jacket consisting of the flowable coating material. The coated fiber 40 is stretched by passing the fiber through the non-circular opening of the forming die 110, by which the jacket is formed to the prescribed non-circular sectional shape which is, for example, a square shape. Namely, the fiber is extruded from the extruder 134 in an apparatus 100 and is then molded by reducing the coating part 44 enclosing the circular core 42 to obtain the final fiber 40 having the desired non-circular sectional shape. The coated fiber 40 is thereafter passed through a curing section 140 provided with an introducing heater 142 and is in succession passed through a coating device section 118. The fiber is taken up on a storage winder 116. In Fig., 10 denotes a winder, 10a the axial direction and 10b the radial direction, respectively.