It is an object to provide a water-soluble preflux containing a low-volatile solubilizing agent excellent in performance to dissolve an imidazole compound in water and capable of bringing out an excellent film-forming property of the imidazole compound and also to provide a treating method for the surface of a metal conductive part which comprises bringing the surface into contact with the above water-soluble preflux. Another object is to provide a printed wiring board wherein the surface of a metal conductive part has been brought into contact with the above water-soluble preflux and a soldering method which comprises bringing the surface of a metal conductive part into contact with the above water-soluble preflux and subsequently soldering the surface. A water-soluble preflux comprisingan imidazole compound and a carboxylic acid compound having 4 to 16 carbon atoms represented by the following general formula 1: wherein R 1 represents a linear or branched alkyl group having 1 to 4 carbon atoms, R 2 represents a hydrogen atom or a methyl group, m represents an integer of 0 to 3, and n represents 1 or 2.