This invention provides a method of producing a porous cross-linked polymer by horizontal continuous polymerization of a water-in-oil higher internal phase emulsion (HIPE), which includes using a weir at the opposite end parts in the width direction of an emulsion supporting member for use in the polymerization. This invention also provides a method of producing a porous cross-linked polymer by horizontal continuous polymerization of an HIPE, which includes using a supporting member of the lower side of the emulsion and a sheet material on the upper side of the emulsion, and performing the polymerization by exerting a tensile force in the width direction and/or the travel direction of the sheet material. This invention further provides a method of producing a porous cross-linked polymer by horizontal continuous polymerization of an HIPE, which includes regulating the thickness of the HIPE by passing the HIPE between a movable supporting member and an object disposed so as to retain a prescribed gap from the movable supporting member.