A substance detection device includes an illuminator that illuminates a monitoring range with light at a first wavelength and light at a second wavelength at different timings, an image capturer that obtains a first actual image by capturing an image of the monitoring range which is illuminated by the light at the first wavelength and obtains a second actual image by capturing an image of the monitoring range which is illuminated by the light at the second wavelength, and an image processor that acquires a difference in lightness of corresponding pixels between the first actual image and the second actual image that are obtained by the image capturer, compares the acquired difference in lightness of the corresponding pixels with a reference value, and detects a specific substance that is present in the monitoring range based on a result of the comparison.