Hydrates have peculiar crystal structure that one guest molecule is surrounded with a cage composed by water molecules.These water molecules are connected by hydrogen bonding each other, but there is no chemical bonding between the guest molecule and the water molecules.In this short monograph I will mention the possibility of hydrate's application to solve environmental problems.The cage-like structure of hydrates can be utilized as an environmental pollutant gas separation method since in mixture hydrates the mole fraction of components in a hydrate phase is different from that in a gas phase.The outline and principle of this technique is considered to be similar to the separation technology using phase equilibrium.Thus firstly the separation technology using phase equilibrium is briefed, then properties of hydrates concerning with the separation technology are summarized concisely.After that the separation technology with hydrates is introduced.