Mixed gas separation method and mixed gas separation apparatus

A mixed gas separation method comprising:

• a) preparing a separation membrane;
• and
• b) supplying a mixed gas that contains a plurality of types of gases to said separation membrane and causing a gas with high permeability in said mixed gas to permeate through said separation membrane to separate said gas with high permeability from said mixed gas,
• wherein in said operation b), when ΔP is a difference between a feed pressure and a permeate pressure, the feed pressure being a gas pressure on a primary side of said separation membrane, the permeate pressure being a gas pressure on a secondary side of said separation membrane, and A is a Joule-Thomson coefficient, a difference ΔT between a feed temperature and a permeate temperature, the feed temperature being a gas temperature on the primary side of said separation membrane, the permeate temperature being a gas temperature on the secondary side of said separation membrane, is made less than 90% of A·ΔP by setting a Nu number in said mixed gas to be greater than or equal to 2 and less than or equal to 10.