A method and apparatus for separating a solution containing a target substance. The solution is atomized into a mist in an atomizer (1) to produce a mixed fluid of mist and gas. In the collection of the mist from this mixed fluid, a gas transmission membrane (51) is used. The gas transmission membrane has a pore size that transmits gas but does not transmit the target substance contained in the mist. The mixed fluid is brought into contact with the primary surface of the gas transmission membrane (51), and the pressure on the primary surface is made higher than the pressure on the secondary surface of the opposite side. Thus, the gas in the mixed fluid is allowed to pass through the gas transmission membrane (51) to separate part or all of the gas contained in the mixed fluid.