A phosphor, an optical device, and a display device are provided. The phosphor includes a composition represented by (Sr(1-x-y)CaxBay)(Ga(1-m-n)AlmInn)aS4:Eu, wherein the concentration of europium is in the range of 1 to 9 mole percent; 0≦x≦1, 0≦y≦1, and x+y≦1; 0≦m≦1, 0≦n≦1, m+n≦1, and 2.9≦a≦5.9; and a main absorption wavelength range of the excitation is selected either a first wavelength range that overlaps the entirety of a reference wavelength range and extends to the shorter wavelength side or a second wavelength range that overlaps the entirety of the reference wavelength range and extends to the longer wavelength side according to the value a.