PURPOSE: To prepare liquefied carbonic acid by simple facilities, to produce water as a by-product, and to suppress a release amount of carbon dioxide gas into atmosphere, by compressing a waste combustion gas of liquefied natural gas, drying, and cooling it so that carbon dioxide is liquefied.

CONSTITUTION: A liquefied natural gas is burned to produce a waste gas in an unsaturated steam state containing almost no sulfur, which is sucked from the flue 1 by the blower 4, introduced into the humidifying and dehumidifying column 5. First, it is brought into contact with the spraying sea water a at the dehumidifying part 5a, cooled and changed into a saturated steam state, it is then subjected to indirect heat exchange at the dehumidifying part 5b with the pure water b previously cooled with the sea water a, it is further cooled to give the condensed pure water c. The waste water is then cooled with a refrigerant by the cooler 6 to about 3W10°C, to give the condensed pure water d. The waste gas in an unsaturated steam state is pressurized by the compressor 7 into a given pressure, passed through the dryer 8 packed with a desiccant, cooled further with a low-temperature liquefied natural gas by the cooler 10, the liquefied carbonic acid 11 is taken out, and the remaining gas e is discharged to the flue 1.