Preferably, the semiconductor layer is made of a nitride-based compound semiconductor (general formula of IniGajAlkN, here 0???i, 0???j, 0???k and i+j+k=1) that can efficiently emit light having a comparatively short wavelength sufficient to excite the fluorescent material. [0054] In the case that a gallium nitride-based compound semiconductor is used, a material such as sapphire, spinel, SiC, Si,