The appearance of wood is known to degrade when the wood is exposed to light. However, as the type and degree of degradation vary with the wavelength of the irradiated light, mechanisms such degradation are not fully understood. One of reason that it is difficult to elucidate the degradation mechanism caused by the irradiation light is that degradation due to factors other than the irradiation light occurs during sample preparation. To evaluate the color change caused by spectral light irradiation, samples were prepared to minimize the effects of other factors, such as other factors related to light and heat, the leakage of extractive components, and differences in cell structure. The samples were subjected to spectral light irradiation, and their optical properties were then measured. Both the transmittance and reflectance of wood changed according to the irradiation wavelength. A comparison of the differences before and after treatment showed a decrease in transmittance and reflectance treated with irradiated light in the 279–373 nm wavelength range and an increase in transmittance and reflectance treated with irradiated light in the 404–496 nm range. The position of the negative peak observed in the difference spectrum was not affected by the wavelength of the irradiation, but the position of the positive peak shifted with the irradiation wavelength. An understanding of these simple changes would shed light on the mechanisms of degradation in light-irradiated wood.