A superlattice avalanche photodiode with a mesa structure is provided, which enables to keep its noise characteristic low for a long time without decreasing the dynamic range for high-speed response. The photodiode has a semiconductor superlattice carrier multiplication layer, a first semiconductor electric-field buffer layers of a first conductivity type formed on one side of the carrier multiplication layer, and a second semiconductor electric-field buffer layer of a second conductivity type formed on the other side of the carrier multiplication layer. The photodiode further has a semiconductor light-absorbing layer of the second conductivity type formed on the second electric-field buffer layer. At least the light-absorbing layer constitutes the mesa structure. The first electric-field buffer layer is made of a central part and a peripheral part surrounding the central part. The peripheral part has a greater electric-field relaxation effect than that of the central part. The interface damage of the light-absorbing layer is restrained through the difference of the electric-field relaxation effect.