To diminish a fault of sealing of liquid by a method wherein internal pressure of an openable hermetically-sealed vessel is reduced by evacuation, a liquid is made to flow down from a liquid holding means inside the vessel to the liquid holding means at the bottom of the vessel and, after it is left as it is for a prescribed time, a gas is introduced into the vessel from the outside.

An openable airtight vessel 101 is sealed up by a large cap 102 and subjected to pressure reduction by switching a selector valve 103 to a vacuum pump 105 side. After the pressure reduction, a Ga alloy 108 introduced into an alloy reservoir 109 is made to flow down into an alloy reservoir 112. After the Ga alloy 108 of a prescribed quantity is introduced into the alloy reservoir 112, it is left as it is for a prescribed time so as to remove a gas or a minute bubble from inside the Ga alloy 108. Then, a gas is introduced into the vessel 101 from the outside by switching the selector valve 103 to the leak valve 106 side. Consequentially, the Ga alloy 108 is filled in a glass thin tube being a material 114 of a thermometer. By increasing the pressure inside the vessel 101, the time for filling is shortened. According to this constitution, it is possible to diminish a fault of sealing of a liquid and to improve the productivity.