In order to improve reliability by preventing an edge breakdown in a semiconductor photodetector having a mesa structure such as a mesa APD, the semiconductor photodetector comprises a mesa structure formed on a first semiconductor layer of the first conduction type formed on a semiconductor substrate, the mesa structure including a light absorbing layer for absorbing light, an electric field buffer layer for dropping an electric field intensity, an avalanche multiplication layer for causing avalanche multiplication to occur, and a second semiconductor layer of the second conduction type, wherein the thickness of the avalanche multiplication layer at the portion in the vicinity of the side face of the mesa structure is made thinner than the thickness at the central portion of the mesa structure.