The invention provides a laser hardening tool having little laser output loss. The laser hardening tool 100 condenses the irradiated beams LB.sub.1 from a semiconductor laser stack 120 via a condensing optical lens 134, and creates parallel light beams LB.sub.3 via a diverging optical lens 142. The laser beams LB.sub.4 having been bent by a first mirror 160 travels through an optical waveguide 150 having a rectangular cross-sectional shape. The laser beams LB.sub.5 having been condensed by a re-condensing optical lens 170 are irradiated through a nozzle 190 to subject a work to hardening. The number of reflections of the laser beams LB.sub.4 in the angular optical waveguide 150 is small, and thus the loss of laser output is reduced.