Abstract: An optical coupler 13, a reflection attenuation amount measurement photodetector 14, and an APC photodetector 15 are provided in a variable-wavelength light source apparatus 1 and the reflection attenuation amount can be measured simply by connecting a device under test without using any external optical power meter, etc, and when wavelength calibration is executed, an external wavelength calibration gas cell 18 and a total reflection termination 20 are connected, whereby the wavelength of an optical signal output from a variable-wavelength light source 11 can be measured and controlled with higher accuracy.