Based on the previous investigations of the cold electron emission from porous silicon and porous polycrystalline silicon (PPS) diodes, we have fabricated PPS diodes with a porosity multilayer structure and evaluated the emission characteristics. The multilayer structure has functions of reforming the electric field distribution and improving the thermal conductivity of the PPS layer. It is demonstrated here that the multilayered sample exhibits high emission current density of similar to0.8 mA/cm(2) and high efficiency of similar to1%, which are close to values required for practical use. The emission is quite stable even with a high emission current density. Ballistic electron emission mechanism is discussed in relation to the energy distribution of emitted electrons.