To solve the problems when collecting a gas from a gas hydrate sedimentary layer, especially the gas hydrate sedimentary layer as a resource.

The method for collecting the decomposed gas obtained by injecting an aqueous solution of an additive having decomposition-promoting effects of the gas hydrate to the gas hydrate sedimentary layer includes increasing a gas decomposition rate by regulating the flowing quantity of the injected aqueous solution of the additive to the sedimentary layer to a proper rate when the decomposition rate from the gas hydrate sedimentary layer to the gas is required to be increased, and decreasing the gas decomposition rate by regulating the flowing quantity of the injected aqueous solution of the additive to the sedimentary layer to an excessive rate when the decomposition rate to the gas is required to be decreased. The apparatus therefor is also provided.