Methods to evaluate the light degradation environment of automotive interior materials in the market have been studied. The aim for this study was calculating the light degradation environment quantitatively on the basis of meteorologically observed value. For the quantitative light degradation factor, Equivalent Total Sunshing Energy (ESE) was used. ESE was a value presented by the authors in 1982. Although ESE correlates closely with the degradation, it takes more than one year to determine at a given resion. With this new method, a monthly average daily ESE at the gien resion was estimated numetically based on the meteorologically observed monthly average solar radiation and monthly maximum air temperature at the resion. The calculated value in relation to actual measured value were accurate to within 10%.