Impurity band structure of boron-doped diamond is investigated using p + / p − / p + mesa structures with thin p − layers, where p + is heavily doped diamond with variable range hopping conductivity and p − is less-doped diamond with valence-band conductivity. From the frequency and temperature dependence of the conductivity of the mesa structures, it is found that holes injected into the p − layer stay at the 2p states of boron for an extremely long time and contribute to the conductivity. From the equivalent circuit analysis it is concluded that, when the boron concentration exceeds ϳ4 ϫ 10 18 cm −3 , the 2p wave functions of boron begin to overlap, and an impurity band forms at 2p states, which is located ϳ0.07 eV above the valence-band maximum. Simultaneously, the Fermi level rises to the impurity band by a strong electron-optical phonon interaction and a variable range hopping is realized in this band.