To improve wavelength measurement accuracy, in the case of using SLD with much light intensity fluctuation as a light source.

In a wave length measuring device, a reflection light of cast light of the light source from an optical fiber Bragg diffraction grating is introduced into an array waveguide diffraction grating separable into a plurality of wave lengths with small intervals of center wave length and the wave length of the reflection light is measured by using the logarithm of an output ratio of a pair of light reception elements provided in each of two output channel of the array waveguide diffraction grating. The wave length measuring device, thus measures the reflection wave length based on a wavelength determination function using an emission light spectrum of the light source, a reflection light spectrum of the optical fiber Bragg diffraction grating, a transmission spectrum of the optical fiber, a light reception sensitivity spectrum of reception element and a transmission spectrum of the two output channel of the array waveguide diffraction grating. The full width at half maximum of the reflection light spectrum of the optical fiber Bragg diffraction grating is increased to a large value until a target accuracy of wave length measurement is obtained.