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 a surrounding area 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, wherein, in the third step, an area obtained by excluding a first expansion area of the detection target area obtained by expanding the contour of the detection target area outwardly by a first expansion amount from a second expansion area obtained by expanding the contour of the detection target area outwardly by a second expansion amount larger than the first expansion amount is set as the peripheral area.