To enable discharging of the formed gas hydrate without discontinuing the formation step to improve its productivity.

In the method for forming a gas hydrate by reacting a stock gas with water in a reaction vessel 1, the reaction vessel 1 is provided with an opening 5a of a withdrawal opening for a gas hydrate, and the formation of the gas hydrate is conducted by setting the liquid level of an aqueous phase L to be formed in the reaction vessel 1 at a position lower than the opening 5a, and when the gas hydrate accumulates on the liquid level of the aqueous phase L, the liquid level is raised to discharge the gas hydrate floating on the aqueous phase L while continuing the above formation.