Method of operating separation apparatus and separation apparatus

A separation apparatus, comprising:

• a separation membrane;
• a supply part for supplying a mixed gas containing a plurality of types of gases to said separation membrane, to thereby separate a substance in said mixed gas, which has high permeability through said separation membrane, from any other substance;
• and
• a control part for controlling said supply part to perform a normal operation for supplying said mixed gas to said separation membrane at a constant set pressure and perform a high pressure treatment for supplying said mixed gas to said separation membrane at a pressure higher than said set pressure at a time of starting supply of said mixed gas to said separation membrane before said normal operation or in a middle of said normal operation.