Light is allowed to be incident from above wells provided on a microplate M and the light transmitted to the lower surface is received to obtain an original image of the wells (Step S101). Detection target areas in the original image are specified by an appropriate image processing (Step S102), and peripheral areas as backgrounds surrounding the respective detection target areas are specified (Step S103). By calculating a density value of the detection target area Ri using luminance information of the detection target area Ri and that of the peripheral area Si surrounding this detection target area Ri for each detection target area Ri (Steps S105, S106), the influence of a well wall surface reflected on the background is eliminated.