NOVELTY - A hydrated source gas is molded to pellet form. Gas-hydrate discharge port (18) of granulator is sealed with liquefied gas (i) in mixer (11). Pellet-form gas hydrate (p) is supplied to mixer, and ejected with gas (i) from mixer and classified. Gas (i) and hydrate (p) are stored in subcooler (12) and lock hopper (13), respectively. Gas (i) and hydrate (p) are supplied to pressure reduction drum (14). Hydrate (p) is overcooled. Hydrate (p) remaining in drum (14) is ejected to storage tank (7). Thus, occurrence of purge gas in gas-hydrate manufacture is prevented.    USE - For preventing occurrence of purge gas during gas-hydrate manufacture.    ADVANTAGE - The method enables effective suppression of purge gas, with suppressed power consumption.    DETAILED DESCRIPTION - A source gas is hydrated with water. The obtained gas hydrate is molded to pellet form, with a granulator. The gas-hydrate discharge port of the granulator is sealed with liquefied gas same as the source gas in a mixer. The pellet-form gas hydrate is supplied from the granulator to the mixer. The liquefied gas and pellet-form gas hydrate are ejected from the mixer, and classified by a classification unit (24). The liquefied gas is stored in a subcooler and the pellet-form gas hydrate is stored in a lock hopper. The liquefied gas cooled to a predetermined temperature by the subcooler, and pellet-form gas hydrate are simultaneously supplied to a pressure reduction drum of pressure release apparatus (5). The pellet-form gas hydrate is overcooled to a predetermined temperature. The pellet-form gas hydrate remaining in pressure reduction drum is ejected to a storage tank. Thus, the occurrence of purge gas during gas-hydrate manufacture is prevented.    DESCRIPTION OF DRAWING(S) - The figure shows the outline block diagram of the pressure release apparatus.    Pressure release apparatus (5)    Storage tank (7)    Mixer (11)    Subcooler (12)    Lock hopper (13)    Pressure reduction drum (14)    Gas-hydrate discharge port (18)    Classification unit (24)    Liquefied gas (i)    Pellet-form gas hydrate (p)