1. A density detection method for detecting a density value of an area having a peculiar optical property within an image, comprising: a first step of irradiating light from above a specimen holding plate including a plurality of light transmissive depressed parts and receiving the light transmitted to a lower side of the specimen holding plate, thereby imaging an original image including the depressed parts; a second step of specifying an area having an optical property different from surrounding as a detection target area from an image area corresponding to the depressed part of the original image; a third step of specifying a peripheral area surrounding the detection target area in the original image and obtaining luminance information of the peripheral area; and a fourth step of calculating a density value of the detection target area based on luminance information of the detection target area and luminance information of the peripheral area corresponding to the target area.