PURPOSE: To utilize a high energy heat source such as laser beam to surely lock a male screw and a female screw at low cost.

CONSTITUTION: This device has laser beam output device 1 for outputting a laser beam, and a converging unit 5 for receiving and converging the laser beam from the laser beam output device 1 through an optical fiber 4, and emitting the converged laser beam to the screwed part of mutually fastened male screw 12 and female screw 11. Each state of fusion or coagulation in the screwed part between the male screw 12 and the female screw 11 by the emission of the laser beam is monitored by a monitor means 16.