As for a water-soluble polymerization starting agent, there include, for example, a persulfate such as potassium persulfate, ammonium persulfate, sodium persulfate and the like; a water-soluble azo compound such as 4,4'-azobis-4-sodium cyanovalerate, 2,2'-azobis(2-amidinopropane)chloride and the like; and hydrogen peroxide.