A semiconductor laser diode comprising a semiconductor substrate having an active layer with a pair of cavity facets opposed to each other on both ends of active layer as well as a first dielectric film of an oxide and a second dielectric film of an oxynitride successively stacked on one cavity facet has sufficient initial characteristics with a film structure having excellent heat radiability for allowing stable high-output lasing over a long period without reducing a catastrophic optical damage level on an emission end.