A point‐like emission mechanism is required for a GaAs polarized electron source to produce an electron beam with high brightness. This is realized by changing the direction of injection laser from a front‐side to a back‐side of the photocathode. Based on this concept, a 20 keV gun (JPES‐1) was constructed and a transmission photocathode including an active layer of a GaAs‐GaAsP superlattice was installed. It produced a laser spot diameter as small as 1.3 μm for the 760∼810 nm laser wavelength, and the brightness of ∼2×107 A⋅cm−2⋅sr−1 corresponding to a reduced brightness of ∼1.0×107 A⋅cm−2⋅sr−1⋅V−1 was obtained for an extracted current of 5.3 μA. This brightness is still smaller than those of W‐field‐emitters, but one order of magnitude higher than those of LaB6 emitters. The peak polarization of ∼90% was achieved at the same time by the photocathode which has an inserted GaAs thin layer between the GaAsP substrate and the GaAaP buffer layer. It demonstrated that the strain‐control of the GaAsP buffer la...