To provide a gas-liquid separation device which enables reduction of tank capacity, and to provide a rear door, a cooling device, and a gas-liquid separation method.SOLUTION: A gas-liquid separation device includes: a tank 13 provided at a heat receiving part which recovers exhaust heat from a cooling object, having a refrigerant steam inflow part 13a, into which refrigerant steam flowing from the heat receiving part through a steam pipe 2 is introduced, and a refrigerant gas outflow part 13b, which discharges a refrigerant gas of the refrigerant steam, and configured to store a refrigerant liquid of the refrigerant steam on a lower surface; and an adjustment valve 12 which floats in the refrigerant liquid stored in the tank 13 and closes a space between the steam pipe 2 and the tank 13.SELECTED DRAWING: Figure 2
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