To provide an apparatus and a method for liquefying gas, which deal with an abrupt change in the amount of gas or a refrigerant.

The gas liquefying apparatus replaces a conventional gas liquefying apparatus with which gas is cooled by a refrigerant into liquefied gas, is equipped with a heat exchanger for carrying out heat-exchange between gas and a refrigerant and at least one of a gas flow rate adjustment valve for regulating a gas flow rate and a liquefied gas flow rate adjustment valve for regulating a liquefied gas flow rate, estimates the reduced amount of gas heat energy based on the states of the gas and the liquefied gas before and after passing through the heat exchanger, estimates the increased amountof refrigerant heat energy based on the states of the refrigerant before and after passing through the heat exchanger and adjusts at least one opening degree of the gas flow rate adjustment valve and the liquefied gas flow rate adjustment valve according to a balance between the reduced amount of the gas heat energy and the increased amount of the refrigerant heat energy.