To provide a light wavelength measuring device and a light wavelength measuring method having a small measurement error.

Intensity modulation is applied to light from a first wide-wavelength band light source 1 and light from a second wide-wavelength band light source 2 respectively with different frequencies F1, F2. A modulated wave from the light source 1 and a modulated wave acquired after transmitting a modulated wave from the light source 2 through a standard wavelength filter 3 are allowed to enter FBG 6a-6c simultaneously, and a signal acquired by performing photoelectric conversion of a reflected wave from the FBG 6a-6c by a light receiving element 12 is separated by each frequency F1, F2. Hereby, a signal light from the FBG 6a-6c and a signal light of the standard wavelength filter 3 can be separated simultaneously, and consequently the influence of nonreproducibility, hysteresis or nonlinearity of a variable wavelength band filter 11 can be reduced, to thereby realize the light wavelength measuring device and the light wavelength measuring method having a small measurement error.