The invention is directed to the provision of a wavelength conversion-type ultraviolet light source apparatus that can obtain a stable output. A light source apparatus according to one mode of the invention includes: a laser light source 1 which produces fundamental light L1; at least one nonlinear optical crystal 3 which takes the fundamental light L1 or a harmonic thereof as incident light and outputs wavelength-converted light L2; and polarization adjusting means 2 which is placed in an optical path of the incident light and causes an output of the wavelength-converted light L2 to change by changing its refractive index for a polarized component of the incident light. The polarization adjusting means 2 changes the amount of change of the refractive index in accordance with an electrical signal output from a photodetector 7.