The changes in the structure and the electronic states of liquid selenium due to the semiconductor-metal (SC-M) transition at high temperatures and pressures are investigated by first-principles molecular-dynamics simulation using generalized-gradient-corrected density functional theory. It is found that the chain structure persists even in the metallic state, though the average length of the chains is decreasing with increasing temperature. From the time changes of the chain structure, it is also found that the interaction between the Se chains is crucially important for bond breaking, and that the bond breaking and the rearrangement of the Se chains occur more frequently at higher temperatures. When the Se-Se bonds break, the anti-bonding states above the Fermi level are stabilized while the non-bonding states below become unstable, and as a result the gap disappears at high temperatures. The eigenstates which fill up the energy gap and give rise to the metallic state of liquid Se have large amplitudes of wavefunctions near the ends of the Se chains. To understand the experimentally observed photo-induced SC-M transition of liquid Se near the triple point, the possibility of inducing bond breaking in a Se chain by exciting an electron in the HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital) is investigated by first-principles molecular-dynamics simulation and such a bond breaking is confirmed.