To provide a semiconductor photo-electric cathode capable of increasing quantum efficiency of forming a semiconductor optical absorption layer with high crystallinity.

This semiconductor photo-electric cathode has a semiconductor optical absorption layer 3 formed on a buffer layer 2 on a substrate 1 and for converting incident light into an electron; and an alkali metal-containing layer 5 formed on the semiconductor optical absorption layer 3 and for emitting the electron into vacuum. The buffer layer 2 has a graded semiconductor layer 2b whose composition is gradually varied so that a lattice constant on the substrate 1 side almost agrees with that of the substrate 1 and a lattice constant on the semiconductor optical absorption layer 3 side almost agrees with that of the semiconductor optical absorption layer 3, and thereby, the crystallinity of the semiconductor optical absorption layer 3 formed on the buffer layer 2 is improved, the life of an electron existing in the semiconductor photo absorption layer 3 is lengthened, and quantum efficiency is enhanced.