The present invention provides a liposome production process which enables production of a liposome that has a nano-size particle diameter and a high water-soluble drug encapsulation rate. The process for producing a W1/O/W2 emulsion of the present invention includes the steps of (1) emulsifying an organic solvent (O) that is a volatile organic solvent, an aqueous solvent (W1) and a mixed lipid component (F1) to give a W1/O emulsion and (2) dispersing the W1/O emulsion produced in step (1) in an aqueous solvent (W2) by use of a porous membrane, to give a W1/O/W2 emulsion, wherein the porous membrane has been surface-treated with a hydrophilic drug so as to allow its surface to have a water contact angle of 0 to 42° in the air.
