A carbon dioxide separation device has an absorption tower for receiving exhaust gas and configured to cause carbon dioxide, included in the exhaust gas, and an absorbent to react with each other. The separation device has a pressure reduction and phase separation unit to depressurize the CO2-rich-solution, and cause heat exchange between a lean solution and the CO2-rich-solution and separate a phase of the CO2-rich-solution into gas and liquid. In the separation device lean solution flows into the pressure reduction and phase separation unit, and the CO2-rich-solution undergoes a phase separation into gas and liquid due to heat of the lean solution, and then, flows into the stripping tower.