PURPOSE: To form protective layers which are vulnerable to bending on the operator's side by changing the length of stepped cylinders, the socket and spigot fitting length of the stepped cylinders, and the difference between the inside diameter in the large diameter cylindrical parts of the stepped cylinders and the outside diameter in the small diameter cylindrical parts thereof thereby making the min. radius of bending of the protective layers for an optical waveguide smaller.

CONSTITUTION: The length L of stepped cylinders 8, the outside diameter D in large diameter cylindrical parts 11, the outside diameter (d) in small diameter cylindrical parts 12 and the socket and spigot fitting length l of the cylinders 8 among the protective layers for an optical fiber as well as the min. redius Rmin of bending of a fiber type optical waveguide for a laser knife and a laser working device have the relation shown by the equation. The protective layers vulnerable to bending on the operator's side of said optical waveguide are obtained by satisfying at least one among the three conditions, i.e., the lengths L, l are made smaller and the difference (D-d) in the diameter is made larger, to make the radius Rmin smaller from the incident end side of light toward the exit end side. Otherwise the outermost circumferential coverings are made gradually thinner toward the exit side. The protective layers which are vulnerable to bending and are easy to operate are thus formed.