PURPOSE: To shorten the time of optical fiber changing operation by providing a small-diameter optical fiber which functions as a buffer between an incidence optical device and a projection optical device.

CONSTITUTION: A 1st small-diameter optical 11 which serves as the buffer is arranged between the incidence optical device 4 and projection optical device 6 for target power transmission. A laser beam is therefore easily connected to a 2nd large-diameter optical fiber 12. Consequently, optical fiber changing operation resulting from the damage of the component on the working end side of an optical fiber device for working, variation in the fiber diameter with working contents, etc., is performed by single operation and the time of the changing operation can be shortened.