We systematically study the electronic properties of two-dimensional group-V materials, i.e. phosphorene, arsenene, antimonene, and bismuthene. The density functional calculations are performed using generalized gradient approximation. We first clarify that the α structure is the most stable in the cases of phosphorene and bismuthene and the β structure is the most stable in the cases of arsenene and antimonene. We next analyze the band structures based on group theory. As a result, we find

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... all the bands are doubly degenerated at the boundaries of the first Brillouin zone in the α structures and the degeneracies are due to the symmetry of the materials. The band gaps of the β structures are found to be larger than those of the α structures. This tendency in the band gaps is explained based on the fact that the band gap increases as the sp hybridization becomes large. We find that buckling occurs in the α structures of antimonene and bismuthene due to the electron transfer from the higher atom to the lower atom.