PURPOSE: To display an optimum range of illumination by calculating plural illumination values and comparing the calculated results with a previously set up value.

CONSTITUTION: Illumination A based upon a main light source 12 (whole lighting) to be measured, illumination B based upon an auxiliary light source 13 (partial lighting) and illumination C free from these light sources 12, 13 are measured respectively. The sum (A+B+C) of illumination and the difference {B-(A+C)} obtained by subtracting the peripheral illumination from the illumination B due to the light source 13 are calculated. These illumination values are compared with the previously set-up illumination range, so that whether proper illumination is applied to a desk 10 or not can be easily decided and the optimum range of illumination can be displayed.