A method comprising liquefying a gas to thereby obtain a liquefied gas; injecting a sample into a mobile phase containing a solvent and a supercritical fluid formed from the liquefied gas; passing this mobile phase through a column so that the mobile phase containing desired substance is divided into the solvent and the gas; and separating the desired substance from the solvent, wherein when the pressure of the gas divided from the mobile phase is higher than the pressure of the gas fed for formation of the liquefied gas from gas supply device, the gas divided from the mobile phase is liquefied. Further, there is provided a vapor liquid separator for use in the method.