There is provided a wavelength determining device including a reference wavelength measuring section 42 that, based upon a number A of interference fringes generated by an optical path difference of first reference wavelength light (wavelength: λ1) and a number C of interference fringes generated by the optical path difference of second reference wavelength light (wavelength: λ2), measures the wavelength of the second reference wavelength light, an input light wavelength measuring section 44, based upon the number A of the interference fringes generated by the optical path difference of the first reference wavelength light (wavelength: λ1) and a number B of interference fringes generated by the optical path difference of input light (wavelength: λx), measures the wavelength of the input light, a correction coefficient determining section 46 that determines a second correction coefficient k based upon the measured wavelength λc of the second reference wavelength light and the measured wavelength λm of the input light, and an input light wavelength correcting section 48 that corrects the measured wavelength of the input light by multiplying the measured wavelength λm of the input light by the second correction coefficient k, in order to correctly measure the wavelength λx(=k·λm) of the input light.