PROBLEM TO BE SOLVED: To provide a photosynthesis promotion light source emitting light suitable for photosynthesis performed in organisms with a photosynthetic pigment, such as plants and algae.SOLUTION: A photosynthesis promotion light source 1 emits light having an emission spectrum that has a peak, where an optical intensity in the entire wavelength region is the maximum, in a wavelength region of 610-630 nm and satisfies relationships of λ1-70 [nm]≤λ2≤λ1-30 [nm] and λ1+30 [nm]≤λ3≤λ1+70 [nm], where λ1 [nm] is the peak wavelength, λ2 [nm] is a wavelength at which the optical intensity is 50% of the optical intensity at the peak wavelength λ1 and which is most closest to the peak wavelength λ1 on a shorter wavelength side than the peak wavelength λ1, and λ3 [nm] is a wavelength which is most closest to the peak wavelength λ1 on a longer wavelength side than the peak wavelength λ1.SELECTED DRAWING: Figure 3
