To supply good-quality gas with stable calories to gas supply destinations, such as power plants, without the need for large-scale facilities.

A target value of gas calorie at the entrance side of a gas main pipe 10 is determined so that the gas calorie at the exit side of the gas main pipe 10 is a predetermined target value during stable operation when neither starting nor stopping a vaporizer occurs. Based on the target value of gas calorie at the entrance side, a mixing ratio of heavy LNG to light LNG to be supplied to the gas main pipe 10 is determined. Based on the mixing ratio, supply amounts of the heavy LNG and the light LNG to each vaporizer during operation are determined. When starting or stopping the vaporizer, the mixing ratio of the heavy LNG to the light LNG to be supplied to the gas main pipe 10 is corrected so as to compensate gas calorie variations of the main pipe 10 caused by the starting or stopping of the vaporizer. Then, the supply amounts of the heavy LNG and the light LNG to each vaporizer during operation are determined based on the mixing ratio after the correction.