A light source apparatus includes an optical plate, a convergence optical system, a transfer optical system and two casting optical systems. The optical plate includes a translucent segment and a fluorescent substance layer. The convergence optical system converges an excitation light to the translucent segment and to the fluorescent substance layer. The transfer optical system transfers a spot of the excitation light to a transfer plane of the optical plate. The first casting optical system projects a light source image of fluorescence, which is formed on the fluorescent substance layer, onto a common casting plane. The second casting optical system which projects the spot of the excitation light onto the common casting plane. Optical path lengths of the two casting optical systems are the same.