The white chromaticity, in which the illumination environment is considered, is measured for each individual color monitor (100). A reference body (Q), comprising a perfect diffuser, is applied on the screen of the color monitor (100). By a test pattern display means (210), a test pattern (T) is displayed on the screen of the color monitor (100) based on tone values of the three primary colors R, G, and B that are stored in a tone value storage means (220). While fixing the tone value of the primary color R at 255, the tone values of the primary colors G and B are varied cyclically from 0 to 255 in a prescribed period by a tone value varying means (240). An operator inputs the result of comparing the color of the test pattern (T) and the color of the reference body (Q). When a comparison result indicating the matching of the two is input, a comparison result entering means (230) outputs an agreement signal. A measurement result output means (250) outputs, as the measurement results indicating the white chromaticity based on the reference body (Q), the tone values of the three primary colors R, G, and B that are stored in the tone value storage means (220) at the point at which the agreement signal is provided.