PURPOSE: To deal with sudden variation of the liquid level in a tank by opening a discharge valve up to the maximum controlled valve opening in the case of the present liquid level higher than the maximum controlled liquid level, meanwhile closing it up to the minimum controlled valve opening in the case of the present liquid level lower than the minimum controlled liquid level.

CONSTITUTION: The discharge side of a cargo pump 1 connected to a vapor-liquid separation tank 2 on the suction side is connected to a discharge valve 3, the valve opening of the discharge valve 3 is opened/closed step by step according to the liquid level in the tank 2, and hence the level in the tank 2 is kept within a decided range. At controlling the level, after detecting the present level of the tank 2, it is judged whether the present level X is over the maximum controlled level or under the minimum controlled level, and when it is over the maximum controlled level, the present opening of the discharge valve is set at the maximum controlled opening, meanwhile when it is under the minimum controlled level, it is controlled to be the minimum controlled opening. When the present level is between the maximum and minimum controlled levels, the valve opening is controlled to increase or decrease step by step.