The protective layer is made of Al1-x-y-zGaxInyBzN (where 0???x, y, z???1 and 0???x+y+z???1), which is transparent to light emitted from the laser diodeIn one embodiment of the present invention, the thickness of the protective layer is preferably N times as large as ??/2n, where N is a positive integer. ?? is an oscillation wavelength of the light emitted from the laser diode and n is a refractiv