PURPOSE: To shorten time until it reaches a specified water spray temperature by calculating a feed-foreward amount from balance between enthalpy taken out from sea water by means of LNG in an evaporator, enthalpy obtained through sea water in a spray receiving bath to which sea water is newly supplied.

CONSTITUTION: A numerator of a calculation formula of a feed-foreward amount is calculated from a sea water temperature T obtained from a temperature gauge 14, known property values of LNG, and a flow rate FLNG of LNG obtained from a flow rate adjustor 8. A denominator is calculated from an inlet sea water temperature TSW obtained from a temperature gauge 15, a target sea water temperature TSV in a spray receiving water bath, and a parameter which is previously set by measuring. The calculated feed-foreward amount is set to a flow rate adjustor 18, and a flow rate adjusting valve 17 is controlled by a sea water rate adjusting loop 19. The sea water temperature spray can be speedily brought to a set value. That is, an LNG evaporator can perform effective control under a normal condition.