Light emitted from first fluorescent layers 30 and second fluorescent layers 31 located on both sides of the central part extending in longitudinal direction of first substrate 22 and second substrate 23 is reflected at an original 43 located on the opposite side of a discharge space of the second substrate 23. Then, the reflected light goes through the central part extending in longitudinal direction of the first substrate 22 and second substrate 23, and is converged into the lens 44 disposed on the opposite side of the discharge space of the first substrate 22. The reflected light thus converged is then detected by a sensor 46. As a result, brightness can be improved due to the light sources being the integrally formed fluorescent layers on both sides, eventually making it possible to reduce the cost.