The discovery of superconducting transition in C₆₀ solid and Si clathrate solid has largely contributed to the development of the studies of cluster-solids. Both solids are packed with cluster of icosahedral symmetry, C₆₀ soccer-ball and Si₂₀ dodecahedron, respectively. Icosahedral boron solids are packed with B₁₂ icosahedral cluster and the only elemental semiconducting solids of group III element. They have topologically similar crystalline structure to those of C₆₀ solid and Si clathrate solid but form peculiar 3-center covalent bonds which may give peculiar electronic structures and properties to the boron solids. In this article we report the electronic-property investigation on metal-doped β-rhombohedral boron (β-rh. B) solid (most commonly known phase among the icosahedral boron solids) and present the feature of the electronic structure. We have investigated electrical conduction, magnetic susceptibility, nuclear magnetic resonance, electron energy loss spectroscopy, and ultra-violet photo-emission spectroscopy for Li-, Cu-, Ni-, and V-doped β-rh. B. Interestingly metal transition was absent even after several at.% of metal-doping, but we found the density of localized states at the Fermi energy to have a peak with respect to metal concentration. Based on the property, we discuss the possibility of filling localized intrinsic acceptor band originated from the uppermost bonding orbital of B₁₂ icosahedral cluster that is split by the Jahn-Teller distortion. In another form of icosahedral boron solids, however, α-rhombohedral boron was predicted to show metal transition after Li doping by band calculation.