To provide a gas replacing method for a low temperature liquefied gas tank without consuming large quantity of gaseous nitrogen, capable of replacing residual gas by gaseous nitrogen in a short time and reducing mixing of gaseous nitrogen in residual gas.

Vaporized gas remaining in an empty low temperature liquefied gas tank 1 from which stored liquid is extracted is replaced with vaporized gas in separate low temperature liquefied gas tank 1' in operation, next, gaseous nitrogen 9 is supplied from the lower part of the empty low temperature liquefied gas tank 1 and residual gas of an upper layer is replaced with gaseous nitrogen of a lower layer by a piston flow.