Optical properties of book paper were measured to evaluate deterioration degrees of books stored in our Faculty Library. About 1, 500 books published in Japan for the past hundred year (1880-1980) were sampled, and the brightness, yellowness, ΔE and K/S values were measured. In these values, a linear relationship between K/S values and root of storage time was obtained, and the storage time of a book can be estimated from its K/S value by using the discoloration rate constant of book paper obtained from the slope of the above line. This optical and non-destructive method can be applied for evaluation of deterioration degrees of book paper.