To provide a method which effectively improves workability in the dissolution of a water-soluble polymer compound in water without the necessity for delicate pH control.

The surface treatment method comprises treating the surface of a water-soluble polymer compound such as cellulose, a polysaccharide, or a derivative thereof with a cyclic acetal compound obtained by the reaction of a polyhydric alcohol with a compound having a plurality of formyl groups to improve the solubility of the water-soluble polymer compound in water. According to the surface treatment method, it is possible to effectively reduce the formation of unwetted polymer particles when the water-soluble polymer is thrown into water.