The present invention, the fibers from the start (1) (2) is released by means of the tension device 5, directed to the die part 7 and the extrusion head 3 is configured by including a tolpi Figure 8 It becomes and, thus relates to a method of coating process that the formed buffer layer to close contact with the primer-coated fibers on the surface of the primer-coated fiber, a close contact with a coated fiber in this method is the extrusion head (3) in a predetermined manner It is cooled in the downstream. The invention also relates also to the structure of the coating process line. First, by means of the correlation between the line tension and the buffer layer peel force determined a target value for the line tension, and measures the line tension between the extrusion head (3) and the tensioning device (5). So that the obtained measurement data, to the line tension and the obtained target value and the measured value is the target value obtained from the comparison, and the line tension measurement by means of a correlation between the buffer layer peel force, the mold portion (the extrusion head (3) in the axial direction of the fiber 7) and / or adjusting the position of Figure tolpi.