Abstract: An apparatus for removing non-condensable gas accumulated in the exchanger tubes of a heat exchanger. A vertically elongated tank having retained in its lower portion a liquefied working medium for the exchanger tubes is provided beside the heat exchanger. The tank communicates as its lower portion with the lower ends of the tubes and at its upper portion with the upper ends of the tubes by lines each provided with a valve. A gas release pipe extends downward from outside into the upper portion of the tank through the tank wall. A working medium condensing cooling tube is provided in the upper portion of the tank. The interior of the tank is partitioned into a condensing tank and a tank main body positioned therebelow and having an upper portion communicating with the upper portion of the condensing tank and a lower portion communicating with the lower portion of the condensing tank.