PURPOSE: To hold the solid components of a slush mixture and the total amt. thereof during the melting of the solid components by holding the wt. of the slush mixture in a principal container to a predetermined numerical value and holding the vol. thereof to a predetermined vol. value.

CONSTITUTION: A detection probe 42 capable of detecting the volumetric change speed of slush hydrogen of a fuel tank 22 by a control mechanism 40, for example, a microprocessor is provided. The control mechanism 40 can generate the control signal proportional to the detected speed of the volumetric change of slush hydrogen of the fuel tank 22. The control mechanism 40 is further equipped with control signal wiring paths 44, 46 related to pumps 36, 38 capable of regulating the respective pumping speeds of pumps 36, 38 proportional to the speed of the volumetric change of slush hydrogen of the fuel tank 22.