In the semiconductor laser module testing device, a temperature control power source changes a temperature of a wavelength locker module, and a wavelength monitoring bias circuit detects an output of a wavelength monitor in the changed temperature range and computes a correlation between a temperature of a semiconductor laser and a wavelength of light output therefrom. Moreover, the wavelength of the output light is locked by controlling the temperature of the wavelength locker module while feeding back the output of the wavelength monitor by a wavelength feedback circuit based on the obtained correlation between the temperature and the wavelength.