To provide a laser irradiation apparatus capable of emitting light having a stabilized intensity from a laser light source.

The laser irradiation apparatus comprises: a laser light source 1; a mirror 3 which reflects a part of light L₁ emitted from the laser light source 1 and transmits the residue; a photodetector 4 which detects reflection light L₂, L₃ from the mirror 3; and a feedback control circuit 5 which receives a signal output from the photodetector 4 and controls the output of the laser light source 1 so as to maintain a constant signal strength. Thickness of the mirror 3 is set so that an interval S of a beam spot on the photodetector 4 of the reflection light L₂ reflected on the front surface of the mirror 3 and a beam spot on the photodetector 4 of the reflection light L₃ reflected on the back surface of the mirror 3 is equal to or greater than a predetermined value.