To provide a wavelength-variable monochromatic light source which can eliminate the problem of chromatic aberrations, while using a lens in an output optical system and which can adjust the irradiated area without decreasing the amount of light.

The zoom lens (lens system 18) is used in the output optical system 3 of the wavelength variable monochromatic light source. There are provided a zoom-driving mechanism 23 for changing the focal length of the lens system 18 and a focusing mechanism 22 for performing focal point adjustment, by moving the overall or a part of the lens system 18 with respect to the optical axis direction thereof. Further, for the automatic control therefor, there are provided a half mirror 19, an image sensor 20, and a controller 21. Accordingly, chromatic aberrations can be eliminated by readjusting the focal point when the output wavelength is varied, and the irradiated area can be adjusted by the zooming.