A light detecting device includes: an optical filter (2) that transmits a first wavelength light having a wavelength in a first wavelength range, a second wavelength light having a wavelength in a second wavelength range, . . . , and an n-th wavelength light having a wavelength in an n-th wavelength range (n is an integer); an optical sensor (3) that detects at least one of a first wavelength light intensity of the first wavelength light, a second wavelength light intensity of the second wavelength light, . . . , and an n-th wavelength light intensity of the n-th wavelength light; and an analysis unit (4) that estimates a light intensity of light having a wavelength in a wavelength range other than at least one of the first wavelength range, the second wavelength range, . . . , and the n-th wavelength range based on at least one of the first wavelength light intensity, the second wavelength light intensity, . . . , and the n-th wavelength light intensity. A correlative relationship exists between a light intensity of light having a wavelength in the at least one wavelength range and the light intensity of the light having the wavelength in the wavelength range other than the at least one wavelength range.