PURPOSE: To improve the reliability of an optical fiber without sticking dust to the end face of the optical fiber by injecting dry gas from a gas jetting nozzle into the inside of an air flow room, and emitting a laser beam while exhausting dry gas from a discharge port.

CONSTITUTION: An air flow room 8 is provided between the output terminal 5 of an optical fiber 3 and a condenser lens 6. Further, one gas jetting nozzle 9 or more is (are) provided in the air flow room 8 so that the dry gas being supplied from the outside is blown into the air flow room 8. Also, one discharge port 10 or more is (are) provided so that blown dry gas is exhausted from the air flow room 8. The gas jetting nozzle 9 is equipped with a choke to make a jetting velocity of the dry gas fast, and the gas jetting nozzle 9 is preferable to be in a tapered truncated cone shape. Consequently, dry gas is always blown into the end face of the optical fiber, the reliability of the device is improved without sticking dust to the end face of the optical fiber, and even if dust is stuck, it is immediately blown out.