1. A detecting device detecting a skin region representing a skin portion of a subject, the detecting device comprising: a first irradiation section that allows the subject to be irradiated with light of a first wavelength; a second irradiation section that allows the subject to be irradiated with light of a second wavelength as a long wavelength in relation to the first wavelength; a captured image creating section that creates a first image on the basis of light reflected from the subject when the subject is irradiated with the light of the first wavelength, creates a second image on the basis of light reflected from the subject when the subject is irradiated with the light of the second wavelength, and creates an external light image on the basis of light reflected from the subject when the subject is not irradiated with any light of the first and second wavelengths; and a detecting section that detects the skin region on the basis of a first value representing a ratio between a difference between a pixel value of the first image and a pixel value of the external light image and a difference between a pixel value of the second image and the pixel value of the external light image or a second value obtained by normalizing the difference between the pixel value of the first image and the pixel value of the second image to at least one of a difference obtained by subtracting the pixel value of the external light image from the pixel value of the first image and the difference obtained by subtracting the pixel value of the external light image from the pixel value of the second image.