A blue-ultraviolet on-p-GaAs substrate pin Zn1-xMgxSySe1-y photodiode with high quantum efficiency, small dark current, high reliability and a long lifetime. The ZnMgSSe photodiode has a metallic p-electrode, a p-GaAs single crystal substrate, a p-(ZnSe/ZnTe)m superlattice (m: integer number of sets of thin films), an optionally formed p-ZnSe buffer layer, a p-Zn1-xMgxSySe1-y layer, an i-Zn1-xMgxSySe1-y layer, an n-Zn1-xMgxSySe1-y layer, an n-electrode and an optionally provided antireflection film. Incidence light arrives at the i-layer without passing ZnTe layers. Since the incidence light is not absorbed by ZnTe layers, high quantum efficiency and high sensitivity are obtained.