A semiconductor photocathode comprising: a glass substrate, a SiO 2 layer and an Al X Ga 1-X N layer, with the Al composition ratio X being 0 ≤ X < 1, attached to the glass substrate via the SiO 2 layer; and an alkali metal-containing layer formed on the Al X Ga 1-X N layer, wherein the Al X Ga 1-X N layer includes: a first region adjacent to the alkali metal-containing layer; a second region adjacent to the SiO 2 layer; and an intermediate region located between the first region and the second region, wherein an Al composition ratio X in the second region decreases linear strictly monotonously from the SiO 2 layer to the intermediate region and has a minimum value of X MIN; an Al composition ratio X in the intermediate region decreases linear strictly monotonously from the second region to the first region and has a minimum value of X MIN; an Al composition ratio X in the first region is in a range from 0 to X MIN; and a thickness D1 of the first region is 18 nm or more, wherein the second region has a semiconductor superlattice structure and wherein the intermediate region has a semiconductor superlattice structure, wherein each semiconductor superlattice structure is made up of a plurality of well layers of GaN and barrier layers of Al X Ga 1-X N, wherein the well layers and barrier layers are laminated alternately, wherein each respective pair of an adjacent well layer and barrier layer is defined as a respective unit section, wherein the Al composition ratio X in a respective unit section is given by an average value of the Al composition ratio X in the well layer and the barrier layer of the unit section, wherein the Al composition ratio X in the semiconductor superlattice structure is given as a discrete function of the position in a thickness direction of the Al X Ga 1-X N layer, wherein the average values of the Al composition ratio X of the unit sections decrease linear strictly monotonically with distance from an interface position between the second region and the SiO 2 layer, in the second region and in the intermediate region. a glass substrate, a SiO 2 layer and an Al X Ga 1-X N layer, with the Al composition ratio X being 0 ≤ X < 1, attached to the glass substrate via the SiO 2 layer; and an alkali metal-containing layer formed on the Al X Ga 1-X N layer, wherein the Al X Ga 1-X N layer includes: a first region adjacent to the alkali metal-containing layer; a second region adjacent to the SiO 2 layer; and an intermediate region located between the first region and the second region, wherein a first region adjacent to the alkali metal-containing layer; a second region adjacent to the SiO 2 layer; and an intermediate region located between the first region and the second region, wherein an Al composition ratio X in the second region decreases linear strictly monotonously from the SiO 2 layer to the intermediate region and has a minimum value of X MIN; an Al composition ratio X in the intermediate region decreases linear strictly monotonously from the second region to the first region and has a minimum value of X MIN; an Al composition ratio X in the first region is in a range from 0 to X MIN; and a thickness D1 of the first region is 18 nm or more, wherein the second region has a semiconductor superlattice structure and wherein the intermediate region has a semiconductor superlattice structure, wherein each semiconductor superlattice structure is made up of a plurality of well layers of GaN and barrier layers of Al X Ga 1-X N, wherein the well layers and barrier layers are laminated alternately, wherein each respective pair of an adjacent well layer and barrier layer is defined as a respective unit section, wherein the Al composition ratio X in a respective unit section is given by an average value of the Al composition ratio X in the well layer and the barrier layer of the unit section, wherein the Al composition ratio X in the semiconductor superlattice structure is given as a discrete function of the position in a thickness direction of the Al X Ga 1-X N layer, wherein the average values of the Al composition ratio X of the unit sections decrease linear strictly monotonically with distance from an interface position between the second region and the SiO 2 layer, in the second region and in the intermediate region.