An apparatus for making a gaseous clathrate has a closed reaction vessel to which are fed a reaction gas and a reaction liquid while an interior of the vessel is maintained at a predetermined pressure and a predetermined temperature so that the gaseous clathrate is formed in the vessel. An outlet nozzle on the vessel defines a flow passage opening into the vessel and having an inner wall surface extending between an inner inlet end inside the vessel and an outer outlet end. The passage extends along an axis, is rotation-symmetrical about the axis, and is of decreasing flow cross section from its inner end to its outer end. The inner wall surface is curved in an S-shape from the inner end to the outer end so that pressure in the vessel forces the formed clathrate out through the passage with increasing compression as the flow cross section decreases.