PURPOSE: To prevent an emission to the outside of laser beams due to the damage of an optical fiber by mounting a strain gage to an optical guide containing the optical fiber and monitoring the bend of the optical guide.

CONSTITUTION: Strain gages 2, 3, 4, 5 are bonded extending over the full length of an optical guide 1. The strain gages 2, 3 and 4, 5 are positioned at the positions of the opposite angles. Infrared beams 9 emitted from an infrared beam laser 8 pass through an optical guide 11 to which strain gages are mounted, are condensed in a hand piece 12 for a laser knife and are irradiated to a body to be irradiated 13. Signals from the strain gages are transmitted over a control section 14, and the curvature of the optical guide 11 is measured by a measuring circuit consisting of a bridge circuit. When the curvature exceeds a limited curvature, a shutter 15 is shut or the oscillation of the infrared beam laser 8 is stopped, and an alarm is given to the outside by an alarm device 16.