The present invention provides a method for inhibiting structure II (sII) gas hydrate formation which comprises treating (i) a gas hydrate inhibitor and (ii) at least one additive selected from the group consisting of N-hexyl-N-methyl-pyrrolidinium tetrafluoroborate (HMP-BF4), M-hydroxyethyl-N-methyl-pyrrolidinium tetrafluoroborate (HEMP-BF4), N-ethyl-N-methyl-immidazolium tetrafluoroborate (EMIM-BF4) and N-ethyl-N-methyl-pyrrolidinium tetrafluoroborate (EMP-BF4) in water, which is in contact with a gas mixture of forming structure II gas hydrate.The additives according to the present invention can exhibit the same or greater effects of inhibiting the formation of the structure II gas hydrate through an interaction, even when the amount of a gas hydrate inhibitor is reduced.