Abstract: A laser light source 1 is provided with a first reflection mirror 11, a laser medium 12, an aperture 13, an output mirror 14, a half mirror 15, a light beam diameter adjuster 16, and a second reflection mirror 17, and outputs laser oscillation light 31 reflected by the half mirror 15 to the outside. The main resonator is composed by the first reflection mirror 11 and the output mirror 14 disposed so as to be opposed to each other with the laser medium 12 placed therebetween. The external resonator is composed by the output mirror 14 and the second reflection mirror 17 disposed so as to be opposed to each other. The second reflection mirror 17 is configured such that it gives amplitude or phase variations to respective positions in the section of a light beam when the light is reflected, the second reflection mirror presents an amplitude or phase variation distribution, and determines the transverse mode of the laser oscillation light 31 based on the amplitude or phase variation distribution.