PURPOSE: To measure a degree of viewer's metamerism due to the difference of age of a viewer with an easy method by providing a photodetector having the characteristic of a CIE color matching function, a photodetector having the characteristic of deviation function and a coefficient device which is correlated to age of the viewer.

CONSTITUTION: Filters 15, 18, and 21 in a light receiving part 4 have characteristics adapted to color matching functions (x), λ, (y), λ, (z), and λ, and filters 16, 17, 19, 20, 22, and 23 have characteristics of deviation functions α, λ, β, λ, γ, and γ, and signals of filters 16, 17, 19, 20, 22, and 23 are multiplied by a coefficient on a basis of a value C based on an age A, which is determined by a coefficient device 7, in a multiplier 6, and these signals and outputs of filters 15, 18, and 21 are subjected to addition or subtraction in an adding/subtracting circuit 8, and tristimulus values of objects 1 and 2 in 20-years and 60-years color matching functions are determined. In an operating circuit 13, a degree of color difference of objects 1 and 2, whose colors are matched in the 20-years color matching function, in the 60-years color matching function is calculated.